U.S. patent number 5,132,945 [Application Number 07/475,941] was granted by the patent office on 1992-07-21 for magnetooptical recording medium allowing overwriting with two or more magnetic layers and recording method utilizing the same.
This patent grant is currently assigned to Canon Kabushiki Kaisha. Invention is credited to Eiichi Fujii, Nobuhiro Kasama, Hisaaki Kawade, Tadashi Kobayashi, Yoichi Osato.
United States Patent |
5,132,945 |
Osato , et al. |
July 21, 1992 |
**Please see images for:
( Certificate of Correction ) ** |
Magnetooptical recording medium allowing overwriting with two or
more magnetic layers and recording method utilizing the same
Abstract
A magnetooptical recording medium provided with a first magnetic
layer and a second magnetic layer having a higher Curie point and a
lower coercive force than the first magnetic layer and being
exchange coupled with the first magnetic layer satisfies the
relation: ##EQU1## wherein H.sub.H is the coercive force of the
first magnetic layer; H.sub.L is the coercive force of the second
magnetic layer; M.sub.2 is the saturation magnetization of the
second magnetic layer; h is the thickness thereof; and
.sigma..sub.w is the magnetic wall energy between the first and
second magnetic layers and a method of recording information on the
same.
Inventors: |
Osato; Yoichi (Yokohama,
JP), Kawade; Hisaaki (Atsugi, JP), Fujii;
Eiichi (Yokohama, JP), Kasama; Nobuhiro
(Yokohama, JP), Kobayashi; Tadashi (Yokohama,
JP) |
Assignee: |
Canon Kabushiki Kaisha (Tokyo,
JP)
|
Family
ID: |
27586378 |
Appl.
No.: |
07/475,941 |
Filed: |
January 30, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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071190 |
Jul 8, 1987 |
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Foreign Application Priority Data
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Jul 8, 1986 [JP] |
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61-158787 |
Aug 16, 1986 [JP] |
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61-191202 |
Nov 5, 1986 [JP] |
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61-262034 |
Nov 25, 1986 [JP] |
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61-278566 |
Nov 25, 1986 [JP] |
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61-278567 |
Feb 2, 1987 [JP] |
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62-20384 |
Feb 3, 1987 [JP] |
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62-21675 |
Feb 4, 1987 [JP] |
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62-23993 |
Feb 6, 1987 [JP] |
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62-24706 |
Feb 6, 1987 [JP] |
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62-24707 |
Feb 10, 1987 [JP] |
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62-27083 |
Feb 10, 1987 [JP] |
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62-27982 |
Feb 23, 1987 [JP] |
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62-33736 |
Mar 10, 1987 [JP] |
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62-52897 |
Mar 16, 1987 [JP] |
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62-70279 |
Mar 26, 1987 [JP] |
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62-70273 |
Mar 26, 1987 [JP] |
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62-70274 |
Mar 26, 1987 [JP] |
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62-70278 |
Mar 26, 1987 [JP] |
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61-72559 |
Jun 18, 1987 [JP] |
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62-153108 |
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Current U.S.
Class: |
369/13.38;
G9B/11.018; G9B/11.051; G9B/11.049; G9B/11.012; 360/59; 365/122;
369/13.49; 369/13.51 |
Current CPC
Class: |
G11B
11/10506 (20130101); G11B 11/10591 (20130101); G11B
11/10519 (20130101); G11B 11/10586 (20130101) |
Current International
Class: |
G11B
11/00 (20060101); G11B 11/105 (20060101); G11B
013/04 () |
Field of
Search: |
;369/13,14
;360/59,224,131 ;365/10,27,32,122 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0217067 |
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Apr 1987 |
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EP |
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0225151 |
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Jun 1987 |
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EP |
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3619618 |
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Dec 1986 |
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DE |
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57-70653 |
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May 1982 |
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JP |
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58-50639 |
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Mar 1983 |
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JP |
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58-08045 |
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Jun 1983 |
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JP |
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60-05404 |
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Jan 1985 |
|
JP |
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61-240453 |
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Oct 1986 |
|
JP |
|
2110459 |
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Jun 1983 |
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GB |
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Other References
IEEE Transactions on Magnetics, vol. 22, No. 5 (Sep., 1986 931:3
.
Mizutani, Japanese Patent Abstracts, vol. 9, No. 86 (p.349). .
Tanaka, Japanese Patent Abstracts, vol. 6, No. 34 (p. 104). .
Tsumashima et al., IEEE Trans. on Mag. vol. Mag 17, No. 6, (Nov.
1981) pp. 2840-2842. .
Mizutani, Japanese Patent Abstracts, vol. 9, No. 86 (Apr., 1985) p.
349. .
Nippon Kogaku K. K., "Overwrite System of Mag. Op. Disk Sys.".
.
Inter Symposium on Mag Op. 4/1987, 7 pages. .
Kobayashi, et al., Japanese Journal of Applied Physics, vol. 20,
No. 11 (Nov. 1981), pp. 2089-2095..
|
Primary Examiner: Levy; Stuart S.
Assistant Examiner: Nguyen; Hoa
Attorney, Agent or Firm: Fitzpatrick, Cella, Harper &
Scinto
Parent Case Text
This application is a continuation of application Ser. No. 071,190,
filed Jul. 8, 1987 now abandoned.
Claims
What is claimed is:
1. A magnetooptical recording medium comprising a first magnetic
layer and a second magnetic layer having a higher Curie point and a
lower coercive force than those of said first magnetic layer and
being exchange coupled with said first magnetic layer,
characterized in that provided between said first magnetic layer
and said second magnetic layer is an adjusting layer for adjusting
a magnetic wall energy .sigma..sub.w between said first and second
magnetic layers such that the magnetic wall energy .sigma..sub.w
satisfies a relation:
wherein H.sub.H is the coercive force of the first magnetic layer;
H.sub.L is the coercive force of the second magnetic layer; M.sub.s
is the saturation magnetization of the second magnetic layer; and h
is the thickness of said second magnetic layer.
2. A magnetooptical recording medium according to claim 1, wherein
said adjusting layer is composed of a material selected from Ti,
Cr, Al, Ni, Fe, Co, rare earth elements and transition metals and
has a thickness in a range from 5 to 50 .ANG..
3. A magnetooptical recording medium according to claim 1, wherein
said first and second magnetic layers are composed of alloys
selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo,
TbFeCo, GdTbCo and GdTbFeCo.
4. A magnetooptical recording medium according to claim 1, wherein
the Curie points of said first and second magnetic layers are
respectively in ranges from 70.degree. to 180.degree. C. and from
150.degree. to 400.degree. C.
5. A magnetooptical recording medium according to claim 1, wherein
the coercive forces of said first and second magnetic layers are
respectively in ranges from 3 to 100 KOe, and from 0.5 to 2
KOe.
6. A magnetooptical recording medium comprising a first magnetic
layer and a second magnetic layer having a higher Curie point and a
lower coercive force than those of said first magnetic layer and
being exchange coupled with said first magnetic layer,
characterized in that provided between said first magnetic layer
and said second magnetic layer is an adjusting layer for adjusting
a magnetic wall energy .sigma..sub.W between said first and second
magnetic layers such that the magnetic wall energy .sigma.W
satisfies a relation:
wherein H.sub.H is the coercive force of the first magnetic layer;
H.sub.L is the coercive force of the second magnetic layer; M.sub.s
is the saturation magnetization of the second magnetization layer;
and h is the thickness of said second magnetic layer, said
adjusting layer being composed of a magnetic material exhibiting
surfacial magnetic anisotropy at room temperature and vertical
magnetic anisotropy at temperatures close to the Curie point of
said first magnetic layer.
7. A magnetooptical recording medium according to claim 6, wherein
said first magnetic layer, second magnetic layer and adjusting
layer are composed of alloys of rare earth element and transition
metal, in which the first magnetic layer has a composition rich in
the transition metal compared with the compensation composition,
said second magnetic layer and adjusting layer have compositions
both rich in the rare earth element compared with the compensation
compositions.
8. A magnetooptical recording medium provided, in succession on a
substrate, with a first magnetic layer with a high Curie point
T.sub.H1 and a low coercive force H.sub.L1, a second magnetic layer
with a lower Curie point T.sub.L2 and a higher coercive force
H.sub.H2 compared with those of said first magnetic layer, and a
third magnetic layer with a higher Curie point T.sub.H3 and a lower
coercive force H.sub.L3 compared with those of said second magnetic
layer; wherein said three magnetic layers are mutually so coupled
as to satisfy conditions: ##EQU11## wherein .sigma..sub.w12 is the
magnetic wall energy of the first and second magnetic layers;
.sigma..sub.w23 is the magnetic wall energy of the second and third
magnetic layers; h.sub.1, h.sub.2 add h.sub.3 are respective
thicknesses of the first, second and third magnetic layers; and
M.sub.s1, M.sub.s2 and M.sub.s3 are respectively saturation
magnetizations of said layers.
9. A magnetooptical recording medium according to claim 8, wherein
the Curie points and coercive forces of said first and third
magnetic layers satisfy relations:
10. A magnetooptical recording medium according to claim 8, wherein
the Curie points of said three magnetic layers are respectively in
a range of 150.degree. to 400.degree. C. for T.sub.H1, 70.degree.
to 200.degree. C. for T.sub.H2, and 100.degree. to 250.degree. C.
for T.sub.H3.
11. A magnetooptical recording medium according to claim 8, wherein
the coercive forces of said three magnetic layers are respectively
in a range of 0.1 to 1 KOe for H.sub.L1, 2 to 10 KOe for H.sub.H2,
and 0.5 to 4 KOe for H.sub.L3.
12. A magnetooptical recording medium according to claim 8, wherein
said first, second and third magnetic layers are composed of alloys
of rare earth element and transition metal, wherein the first and
second magnetic layers have compositions both rich in the
transition metal compared with the compensation composition, while
the third magnetic layer has a composition rich in the rare earth
element compared with the compensation composition.
13. A magnetooptical recording medium according to claim 8, wherein
said first, second and third magnetic layers are composed of alloys
selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo,
TbFeCo, and GdTbCo.
14. An information recording process on a magnetooptical recording
medium provided, in succession on a substrate, with a first
magnetic layer with a high Curie point T.sub.H1 and a low coercive
force H.sub.L1. a second magnetic layer with a lower Curie point
T.sub.L2 and a higher coercive force H.sub.H2 compared with those
said first magnetic layer, and a third magnetic layer with a higher
Curie point T.sub.H3 and a lower coercive force H.sub.L3 compared
with those of said second magnetic layer, wherein said three
magnetic layers are mutually so coupled as to satisfy conditions:
##EQU12## wherein .sigma..sub.w12 is the magnetic wall energy of
the first and second magnetic layers: .sigma..sub.w23 is the
magnetic wall energy of the second and third magnetic layers;
h.sub.1, h.sub.2 and h.sub.3 are respective thicknesses of the
first, second and third magnetic layers; and M.sub.s1, M.sub.s2 and
M.sub.s3 are respectively saturation magnetizations of said layers,
comprising steps of: orienting the magnetization of the third
magnetic layer in a predetermined direction while retaining the
magnetization of the second magnetic layer; and
(b) selectively effecting either a first type recording by
irradiating said medium with a light beam of a power for heating
the medium close to the Curie point T.sub.L2 of the second magnetic
layer while applying a bias magnetic field as a bias field, thereby
orienting the magnetizations of the first and second magnetic layer
in a stable direction with respect to the magnetization of the
third magnetic layer while retaining the magnetization of the third
magnetic layer, or a second type recording by irradiating the
medium with a light beam of a power for heating the medium close to
the Curie point T.sub.H3 of the third magnetic layer, thereby
inverting the magnetization of said third magnetic layer and
simultaneously orienting the magnetizations of the first and second
magnetic layers in a stable direction with respect to the
magnetization of the third magnetic layer, according to information
signal.
15. A magnetooptical recording medium provided, in succession on a
substrate, with a first magnetic layer with a Curie point T.sub.1
and a coercive force H.sub.1, a second magnetic layer with a Curie
point T.sub.2 and a coercive force H.sub.2 and a third magnetic
layer with a Curie point T.sub.3 and a coercive force H.sub.3.
which comprises satisfying conditions:
(A) that each magnetic layer is composed principally of an
amorphous alloy of rare earth element and transition metal;
(B) that
(C) that the first magnetic layer has a composition rich in the
transition metal compared with the compensation composition and the
second and third magnetic layers have compositions both rich in the
rare earth element, or that the first magnetic layer has a
composition rich in the rare earth element and the second and third
magnetic layers have compositions both rich in the transition
metal.
16. A magnetooptical recording medium according to claim 15,
wherein the Curie points of said three magnetic layers are
respectively in the ranges of 150.degree. to 400.degree. C. for
T.sub.3, 70.degree. to 200.degree. C. for T.sub.1 and 90.degree. to
400.degree. C. for T.sub.2.
17. A magnetooptical recording medium according to claim 15,
wherein the coercive forces of said three magnetic layers are
respectively in the ranges of 0.1 to 1 KOe for H.sub.2, 2 to 10 KOe
for H.sub.1, and 0.5 to 4 KOe for H.sub.3.
18. An information recording process on a magnetooptical recording
medium provided, in succession on a substrate, with a first
magnetic layer with a Curie point T.sub.1 and a coercive force
H.sub.1, a second magnetic layer with a Curie point T.sub.2 and a
coercive force H.sub.2 and a third magnetic layer with a Curie
point T.sub.3 and a coercive force H.sub.3, and satisfying
conditions:
(A) that each magnetic layer is composed principally of an
amorphous alloy of rare earth element and transition metal;
(B) that H.sub.1 >H.sub.3 >H.sub.2 and T.sub.3
.gtoreq.T.sub.2 >T.sub.1 ; and
(C) that the first magnetic layer has a composition rich in the
transition metal compared with the compensation composition while
the second and third magnetic layers have compositions both rich in
the rare earth element, or that the first magnetic layer has a
composition rich in the rare earth element while the second and
third magnetic layers have compositions both rich in the transition
metal, comprising steps of:
(a) orienting the magnetization of the second and third magnetic
layers in a predetermined direction while retaining the
magnetization of the first magnetic layer; and
(b) selectively effecting either a first type recording by
irradiating said medium with a light beam of a power for heating
the medium close to the Curie point T.sub.1 of said first magnetic
layer while applying a bias magnetic field as a bias field, thereby
orienting the magnetization of the first magnetic layer in a stable
direction with respect to the magnetizations of the second and
third magnetic layer while retaining the magnetizations of the
second and third magnetic layers, or a second type recording by
irradiating the medium with a light beam of a power for heating the
medium close to the Curie point T.sub.3 of the third magnetic layer
thereby inverting the magnetization of said second and third
magnetic layers and simultaneously orienting the magnetization of
the first magnetic layer in a stable direction with respect to the
magnetizations of the second and third magnetic layers, according
to information signal.
19. A magnetooptical recording medium provided, on at least a
substrate, with a quadraple-layered magnetic film consisting of a
first magnetic layer with a Curie point T.sub.1, a coercive force
H.sub.1, a thickness h.sub.1 and a saturation magnetization
M.sub.s1, a second magnetic layer with a Curie point T.sub.2, a
coercive force H.sub.2, a thickness h.sub.2 and a saturation
magnetization M.sub.s2, a third magnetic layer with a Curie point
T.sub.3, a coercive force H.sub.3, a thickness h.sub.3 and a
saturation magnetization M.sub.s3, and a fourth magnetic layer with
a Curie point T.sub.4, a coercive force H.sub.4, a thickness
h.sub.4 and a saturation magnetization M.sub.s4, in which said
magnetic layers are exchange-coupled, wherein said four magnetic
layers are so coupled as to satisfy conditions:
(I) as for the Curie points of the magnetic layers:
(II) as for the coercive forces of the magnetic layers:
(III) as for the thicknesses of the magnetic layers:
(IV) as for the saturation magnetizations, thicknesses, coercive
forces and magnetic wall energies of the magnetic layers: ##EQU13##
wherein .sigma..sub.w12, .sigma..sub.w23 and .sigma..sub.w34 are
magnetic wall energies respectively for the first and second
magnetic layers, second and third magnetic layers, and third and
fourth magnetic layers.
20. An information recording process on a magnetooptical recording
medium provided, at least on a substrate, with a quadraple-layered
magnetic film consisting of a first magnetic layer with a Curie
point T.sub.1, a coercive force H.sub.1, a thickness h.sub.1 and a
saturation magnetization M.sub.s1, a second magnetic layer with a
Curie point T.sub.2, a coercive force H.sub.2, a thickness h.sub.2
and a saturation magnetization M.sub.s2, a third magnetic layer
with a Curie point T.sub.3, a coercive force H.sub.3, a thickness
h.sub.3 and a saturation magnetization M.sub.s3, and a fourth
magnetic layer with a Curie point T.sub.4, a coercive force
H.sub.4, a thickness h.sub.4 and a saturation magnetization
M.sub.s4, in which said magnetic layer are so exchange-coupled as
to satisfy following conditions:
(I) as for the Curie points of the magnetic layers:
(II) as for the coercive forces of the magnetic layers:
(III) as for the thicknesses of the magnetic layers:
(IV) as for the saturation magnetizations, thicknesses, coercive
forces and magnetic wall energies of the magnetic layers: ##EQU14##
wherein .sigma..sub.w12, .sigma..sub.w23 and .sigma..sub.w34 are
magnetic wall energies respectively for the first and second
magnetic layers, second and third magnetic layers, and third and
fourth magnetic layers; comprising steps of:
(a) orienting the magnetization of the fourth magnetic layer in a
predetermined direction while retaining the magnetization of the
second magnetic layer; and
(b) selectively effecting either a first type recording by
irradiating the medium with a light beam of a power for heating the
medium close to the Curie point T.sub.2 of the second magnetic
layer while applying a bias magnetic field as a bias field, thereby
orienting, across the third magnetic layer, the magnetizations of
the first and second magnetic layers in a stable direction with
respect to the magnetization of the fourth magnetic layer while
retaining the magnetization of the fourth magnetic layer, or a
second type recording by irradiating the medium with a light beam
of a power for heating the medium close to the Curie point T.sub.4
of the fourth magnetic layer thereby inverting the magnetization of
said fourth magnetic layer and simultaneously orienting the
magnetizations of the first, second and third magnetic layers in a
stable direction with respect to the magnetization of said fourth
magnetic layer, according to information signal.
21. A magnetooptical recording medium comprising:
a substrate;
a first magnetic layer formed on said substrate;
a second magnetic layer formed on said first magnetic layer, said
second magnetic layer having a higher Curie point and a lower
coercive force at room temperature than those of said first
magnetic layer; and
a third magnetic layer provided between said first magnetic layer
and said second magnetic layer, said third magnetic layer
exhibiting surfacial magnetic anisotropy at room temperature and
vertical magnetic anisotropy at temperatures close to the Curie
point of said first magnetic layer.
22. A magnetooptical recording medium according to claim 21,
wherein said first, second and third magnetic layers are composed
of alloys of rare earth element and transition metal, and said
first magnetic layer has a composition rich in the transition metal
compared with a compensation composition, while said second and
third magnetic layers have compositions both rich in the rare earth
element compared with the compensation composition.
23. A magnetooptical recording medium according to claim 21,
wherein the magnetic anisotropy of said third magnetic layer varies
in a range from 50.degree. to 100.degree. C.
24. A magnetooptical recording medium according to claim 21,
wherein said third magnetic layer is composed of an alloy of more
than one element selected from Dy, Nb, Pr and Tb and more than one
element selected from Co, Fe and Ni.
25. A magnetooptical recording medium according to claim 21,
wherein said third magnetic layer is composed of rare earth
ortho-ferrite or rare earth ortho-chromite.
26. A magnetooptical recording medium according to claim 21,
wherein said first and second magnetic layers are composed of
alloys selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe,
GdFeCo, TbFeCo, GdTbCo and GdTbFeCo.
27. A magnetooptical recording medium according to claim 21,
wherein the Curie point of said first magnetic layer is in a range
from 70.degree. to 180.degree. C. and the Curie point of said
second magnetic layer is in a range from 150.degree. to 400.degree.
C.
28. A magnetooptical recording medium according to claim 21,
wherein the coercive force of said first magnetic layer is in a
range from 3 to 10 KOe and the coercive force of said second
magnetic layer is in a range from 0.5 to 2 KOe.
29. A magnetooptical recording medium according to claim 21,
wherein a pregroove is provided on said substrate.
30. A magnetooptical recording medium according to claim 21,
further comprising a protective layer provided on said second
magnetic layer and between said first magnetic layer and said
substrate.
31. A process for recording information on a magnetooptical
recording medium which comprises a substrate, a first magnetic
layer formed on the substrate, a second magnetic layer formed on
the first magnetic layer and having a higher Curie point and a
lower coercive force at room temperature than those of the first
magnetic layer and a third magnetic layer provided between the
first and second magnetic layers and exhibiting surfacial magnetic
anisotropy at room temperature and vertical magnetic anisotropy at
temperatures close to the Curie point of the first magnetic layer,
said process comprising the steps of:
(a) orienting the magnetization of the second magnetic layer in a
predetermined direction while retaining the magnetization of the
first magnetic layer; and
(b) selectively effecting either a first type recording by
irradiating the medium with a light beam of a power for heating the
medium close to the Curie point of the first magnetic layer while
applying a bias magnetic field, thereby orienting the magnetization
of the first magnetic layer in a stable direction with respect to
the magnetization of the second magnetic layer while retaining the
magnetization of the second magnetic layer, or a second type
recording by irradiating the medium with a light beam of a power
for heating the medium close to the Curie point of the second
magnetic layer thereby inverting the magnetization of the second
magnetic layer and simultaneously orienting the magnetization of
the first magnetic layer in a stable direction with respect to the
magnetization of the second magnetic layer, according to
information signal.
32. A magnetooptical recording medium comprising:
a substrate;
a first magnetic layer formed on said substrate;
a second magnetic layer formed on said first magnetic layer and
having a lower Curie point and a higher coercive force at room
temperature than those of said first magnetic layer, said second
magnetic layer being exchange-coupled with said first magnetic
layer; and
a third magnetic layer formed on said second magnetic layer and
having a high Curie point and a lower coercive force at room
temperature than those of said second magnetic layer, said third
magnetic layer being exchange-coupled with said second magnetic
layer and the exchange-couple force between said second and third
magnetic layers being small compared with that between said first
and second magnetic layers.
33. A magnetooptical recording medium according to claim 32,
wherein the Curie point of said first magnetic layer is equal to or
higher than that of said third magnetic layer.
34. A magnetooptical recording medium according to claim 32,
wherein the coercive force of said first magnetic layer is equal to
or lower than that of said third magnetic layer.
35. A magnetooptical recording medium according to claim 32,
wherein the Curie points of said first, second and third magnetic
layers are respectively in ranges from 150.degree. to 400.degree.
C., from 70.degree. to 200.degree. C. and from 100.degree. to
250.degree. C.
36. A magnetooptical recording medium according to claim 32,
wherein the coercive forces of said first, second and third
magnetic layers are respectively in ranges from 0.1 to 1 KOe, from
2 to 10 KOe and from 0.5 to 4 KOe.
37. A magnetooptical recording medium according to claim 32,
wherein said first, second and third magnetic layers are composed
of alloys of rare earth metal and transition metal, said first and
second magnetic layers have compositions rich in the transition
metal compared with the compensation composition and said third
magnetic layer has a composition rich in the rare earth compared
with the compensation composition.
38. A magnetooptical recording medium according to claim 32,
wherein said first, second and third magnetic layers are composed
of alloys selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe,
GdFeCo, TbFeCo, GdTbCo and GdTbFeCo.
39. A process for recording information on a magnetooptical
recording medium which comprises a substrate, a first magnetic
layer formed on the substrate, a second magnetic layer formed on
the first magnetic layer, having a lower Curie point and a higher
coercive force at room temperature than those of the first magnetic
layer and exchange-coupled with said first magnetic layer an a
third magnetic layer provided on the second magnetic layer, having
a higher Curie point and a lower coercive force at room temperature
than those of the second magnetic layer and exchange-coupled with
the second magnetic layer, said exchange-couple force between the
second and third magnetic layers being small compared with that
between the first and second magnetic layers, said process
comprising the steps of:
(a) orienting the magnetization of the third magnetic layer in a
predetermined direction while retaining the magnetization of the
second magnetic layer; and
(b) selectively effecting either a first type recording by
irradiating the medium with a light beam of a power for heating the
medium close to the Curie point of the second magnetic layer while
applying a bias magnetic field, thereby orienting the
magnetizations of the first and second magnetic layers in stable
directions with respect to the magnetization of the third magnetic
layer while retaining the magnetization of the third magnetic
layer, or a second type recording by irradiating the medium with a
light beam of a power for heating the medium close to the Curie
point of the third magnetic layer thereby inverting the
magnetization of the third magnetic layer and simultaneously
orienting the magnetizations of the first and second magnetic
layers in stable directions with respect to the magnetization of
the third magnetic layer, according to information signal.
40. A magnetooptical recording medium comprising:
a first magnetic layer;
a second magnetic layer exchange-coupled to said first magnetic
layer, said second magnetic layer having a higher Curie point and a
lower coercive force at room temperature than those of said first
magnetic layer; and
a third magnetic layer provided between said first magnetic layer
and said second magnetic layer, said third magnetic layer having a
larger saturation magnetization than that of said first and second
magnetic layers.
41. A magnetooptical recording medium according to claim 40,
wherein said first, second and third magnetic layers are composed
of amorphous alloys of rare earth element and transition metal
element.
42. A magnetooptical recording medium according to claim 41,
wherein said first magnetic layer is composed of a composition rich
in one of the transition metal element and the rare earth element
and said second and third magnetic layers are composed of the other
of the transition metal element and the rare earth element.
43. A magnetooptical recording medium according to claim 41,
wherein aid first and second magnetic layers are composed of alloys
selected from GdCo, GdFe, TbFe, DyFe, GdTbFe, TbDyFe, GdFeCo,
TbFeCo, GdTbCo and GcTbFeCo.
44. A magnetooptical recording medium according to claim 40,
wherein the Curie points of said first, second and third magnetic
layers are respectively in ranges from 70.degree. to 200.degree.
C., from 150.degree. to 400.degree. C. and from 90.degree. to
400.degree. C.
45. A magnetooptical recording medium according to claim 40,
wherein the coercive forces of said first, second and third
magnetic layers are respectively in ranges from 2 to 10 KOe, from
0.5 to 4 KOe and from 0.1 to 1 KOe.
46. A magnetooptical recording medium according to claim 40,
wherein the following conditions are satisfied: ##EQU15## wherein
.sigma..sub.w13 is the magnetic wall energy between the first and
third magnetic layers; .sigma..sub.w23 is the magnetic wall energy
between the second and third magnetic layers; h.sub.1, h.sub.2 and
h.sub.3 are respectively thicknesses of the first, second and third
magnetic layers; and M.sub.s1, M.sub.s2 and M.sub.s3 are
respectively saturation magnetizations of said layers.
47. A process for recording information on a magnetooptical
recording medium which comprises a first magnetic layer, a second
magnetic layer exchange-coupled to the first magnetic layer and
having a higher Curie point and a lower coercive force at room
temperature than those of the first magnetic layer and a third
magnetic layer provided between the first and second magnetic
layers and having a larger saturation magnetization than said those
magnetic layers, said process comprising steps of:
(a) orienting the magnetization of the second magnetic layer in a
predetermined direction while retaining the magnetization of the
first magnetic layer; and
(b) selectively effecting either a first type recording by
irradiating the medium with a light beam of a power for heating the
medium close to the Curie point of the first magnetic layer while
applying a bias magnetic field, thereby orienting the magnetization
of the first magnetic layer in a stable direction with respect to
the magnetization of the second magnetic layer while retaining the
magnetization of the second magnetic layer, or a second type
recording by irradiating the medium with a light beam of a power
for heating the medium close to the Curie point of the second
magnetic layer thereby inverting the magnetization of the second
magnetic layer and simultaneously orienting the magnetization of
the first magnetic layer in a stable direction with respect to the
magnetization of the second magnetic layer, according to
information signal.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetooptical recording medium
provided with a recording layer composed of a magnetic film
allowing information recording by irradiation with a light beam,
and a recording method utilizing said recording medium.
2. Description of the Related Background Art
Optical memory devices utilizing laser beam are being actively
developed in recent years as useful memories of high density and
large capacity. In particular, magnetooptical recording is
attracting attention as a rewritable recording method, and the
magnetooptical recording media employed as a rewritable optical
memory device.
FIG. 1 schematically illustrates a conventional apparatus for such
magnetooptical recording, wherein a disk-shaped magnetooptical
recording medium or magnetooptical disk 31, provided with a
magnetic layer having an easy axis of magnetization perpendicular
to the surface of said layer, is rotated by a spindle motor 32. An
optical head 34, provided with a laser unit, an objective lens 34
etc., performs information recording by projecting a light beam 35
(turned on and off according to the information to be recorded)
onto the disk 31 through the objective lens 33. A bias magnetic
field is applied by an electromagnet 36 to an area of the disk 31
irradiated by the light beam. The optical head 34 is moved in the
radial direction of the disk 31, thereby recording information in
spiral or concentric patterns.
In conventional apparatus as shown in FIG. 1, the information
recording and erasure are conducted in steps 30a to 30f shown in
FIG. 2. At first, as shown in 30a, the magnetic layer 37,
constituting the recording layer of the magnetooptical disk, is
magnetized in a predetermined direction. Then, as shown in 36b, the
magnetic layer 37 is irradiated by the light beam 35. The
irradiated area is heated close to the Curie point of said magnetic
layer 37 by absorption of the irradiating beam, thus causing a
decrease in the coercive force. In this state the magnet 36 shown
in FIG. 1 applies a bias magnetic field B' of a direction opposite
to the aforementioned predetermined direction, whereby the
magnetization in the area irradiated by the light beam is inverted.
Thus, after having passed the position of irradiation, as shown by
30c, a record bit 38 having a direction of magnetization different
from that in the surrounding area is formed. The information is
recorded as a train of such record bits 38 or an information
track.
For erasing the information recorded as in 30d, an unmodulated
light beam 35 is projected while a bias magnetic field -B' of a
direction opposite to that of the magnetic field at the recording
is applied by the magnet 36 as shown in 30e, thereby heating the
magnetic layer 37 again to a temperature close to the Curie point.
Thus the magnetic layer 37 restores the magnetization aligned in
the predetermined direction, thus returning to the state prior to
the recording as shown in 30f.
The recorded information can also be reproduced by irradiating the
magnetic layer 37 having record bits 38, with an unmodulated light
beam of a reduced intensity insufficient for heating to the Curie
point, and detecting the direction of polarization of the reflected
or transmitted light beam by a known method utilizing
magnetooptical effect.
However, in case of rewriting already recorded information, the
conventional apparatus as explained above is incapable of so-called
overwriting but requires a step of erasing followed by a step of
new recording. Thus, in case of changing the information recorded
in a track on a magnetooptical disk, it becomes necessary to erase
the information of said track in a turn of the disk and to record
the new information in a succeeding turn, and such operation
inevitably results in a loss of recording speed.
In order to resolve such drawback there has already been proposed
an apparatus equipped with a record/reproducing head and a separate
erasing head, or an apparatus in which the recording is achieved by
modulating the applied magnetic field while a continuous laser beam
is projected, but such apparatus are associated with other
drawbacks such as being bulky and expensive or incapable of high
speed modulation.
On the other hand, in order to improve the recording sensitivity
and the S/N ratio at reproduction in such magnetooptical recording
medium, technology utilizing two mutually exchange-coupled magnetic
layers is disclosed in the Japanese Patent laid-open No.
78652/1982, corresponding to the U.S. patent application Ser. No.
315,467 which is continued as Continuation-in-part No. 644,143,
which is further continued as Continuation No. 908,934 now U.S.
Pat. No. 4,799,114 issued Jan. 17, 1989. In addition to the
above-mentioned applications, such magnetic layer of two-layered
structure was described in "Magnetization Process of
Exchange-coupled Ferrimagnetic Double-Layered Films", Kobayashi et
al., Japanese Journal of Applied Physics, Vol. 20. No. 11, November
1981, P. 2089-2095 and "Thermomagnetic Writing on Exchange-coupled
Amorphous Rare-Earth Iron Double-layer Films" Tsunashima et al.,
IEEE Transactions on Magnetics, Vol. MAG-17, No. 6, November 1981,
P. 2940-2842.
However such exchange-coupled double-layered films are still
incapable of overwriting and thus require an erasing step.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a magnetooptical
recording medium and a recording method, which are free from the
above-explained drawbacks and enable an overwriting operation as in
the magnetic recording media, by merely attaching magnetic field
generating means of a simple structure to the conventional
apparatus.
The foregoing object can be achieved, according to the present
invention, by a magnetooptical recording medium composed of a
substrate, a first magnetic layer formed thereon, and a second
magnetic layer exchange-coupled with said first magnetic layer and
having a higher Curie point and a lower coercive force compared
with those of said first magnetic layer, and satisfying a
condition:
wherein M.sub.s is the saturation magnetization of the second
magnetic layer, h is the thickness thereof, .sigma..sub.w is the
magnetic wall energy between two magnetic layers, and H.sub.H and
H.sub.L are coercive forces the first and second magnetic
layers.
The information recording on said medium is conducted by a step of
applying a first magnetic field of a magnitude enough for
magnetizing said second magnetic layer but insufficient for
inverting the direction of magnetization of said first magnetic
layer, and a step of applying a bias magnetic field of a direction
opposite to that of said first magnetic field and simultaneously
projecting a light beam of a power enough for heating the medium
close to the Curie point of the first magnetic layer thereby
obtaining the recording of a first kind in which the magnetization
of the first magnetic layer is aligned in a direction stable to the
second magnetic layer while the direction of magnetization of the
second magnetic layer is not changed, or applying said bias
magnetic field and simultaneously projecting a light beam of a
power enough for heating the medium close to the Curie point of the
second magnetic layer thereby obtaining the recording of a second
kind in which the direction of magnetization of the second magnetic
layer is inverted and the first magnetic layer is simultaneously
magnetized in a direction stable to the second magnetic layer.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic view of a conventional magnetooptical
recording apparatus;
FIG. 2 is a schematic view showing the recording and erasing
processes utilizing a conventional magnetooptical recording
medium;
FIG. 3 is a schematic cross-sectional view of a magnetooptical
recording medium embodying the present invention;
FIG. 4 is a chart showing the temperature characteristic of the
coercive force of the magnetic layer in the medium shown in FIG.
3;
FIG. 5 is a schematic view showing the state of magnetization in a
recording process utilizing the medium shown in FIG. 3;
FIG. 6 is a schematic view of an apparatus for recording and
reproduction with the medium of the present invention;
FIGS. 7A to 7D are charts showing B-H loops of the medium shown in
FIG. 3;
FIG. 8 is a schematic view of an embodiment of the magnetooptical
recording apparatus utilizing the medium of the present
invention;
FIG. 9 is a chart showing the mode of modulation of the light beam
irradiating the medium in the apparatus shown in FIG. 8;
FIG. 10A is a schematic view showing states of magnetization in the
recording process utilizing the apparatus shown in FIG. 8;
FIG. 10B is a schematic view showing states of magnetization in the
recording process utilizing another embodiment of the medium;
FIG. 11 is a chart showing the relation between the coercive force
of the first magnetic layer and the magnetic field inducing noise
increase;
FIGS. 12 and 13 are schematic cross-sectional views of embodiments
of the magnetooptical recording medium provided with an adjusting
layer for the magnetic wall energy;
FIG. 14 is a chart showing the relation between the thickness of
the adjusting layer and the exchange force of the magnetic layers,
in the medium shown in FIG. 13;
FIG. 15 is a chart showing the temperature characteristic of the
effective bias magnetic field of the magnetic layers in the medium
shown in FIG. 13;
FIGS. 16 and 17 are charts showing the temperature characteristic
of the magnetic field required for orienting the magnetization of
the adjusting layer in the perpendicular direction;
FIG. 18 is a chart showing the temperature characteristic of the
coercive force of the magnetic layers in the medium shown in FIG.
3;
FIG. 19 is a chart showing the temperature characteristic of the
coercive force and exchange force of the magnetic layers in the
medium shown in FIG. 3;
FIGS. 20 and 21 are schematic views showing states of magnetization
in the recording process utilizing the compensation
temperature;
FIGS. 22 and 23 are charts showing the temperature characteristic
of the coercive force, when the magnetic layer has the compensation
temperature between the room temperature and the Curie
temperature;
FIGS. 24, 25 and 26 are schematic cross-sectional views of
embodiments of the magnetooptical recording medium of the present
invention provided with a protective layer;
FIGS. 27 and 28 are schematic cross-sectional views of embodiments
of the magnetooptical recording medium of the present invention
utilizing triple-layered magnetic film;
FIGS. 29, 31 to 33 are charts showing states of magnetization in
the recording process utilizing the medium shown in FIG. 27;
FIGS. 30 and 34 are charts showing the temperature characteristic
of the coercive force of the magnetic layers in the medium shown in
FIG. 27;
FIGS. 35 and 36 are schematic cross-sectional views of embodiments
of the magnetooptical recording medium of the present invention
utilizing quadraple-layered magnetic film;
FIG. 37 is a schematic view showing states of magnetization in the
recording process utilizing the medium shown in FIG. 35;
FIG. 38 is a chart showing the temperature characteristic of the
coercive force of the magnetic layers in the medium shown in FIG.
35;
FIGS. 39, 41, 42 and 43 are charts showing states of magnetization
in the recording and erasing processes utilizing the medium shown
in FIG. 3;
FIG. 40 is a chart showing the relation between the erasing laser
power and the residual signal after erasure in the medium shown in
FIG. 3;
FIG. 44 is a chart showing the relation between the recording laser
power and the reproduced C/N ratio in the medium shown in FIG. 3;
and
FIGS. 45 and 46 are schematic views showing variations of the
magnetooptical recording apparatus utilizing the medium of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Now the present invention will be clarified in detail by
embodiments thereof shown in the attached drawings.
FIG. 3 is a schematic cross-sectional view of an embodiment of the
magnetooptical recording medium of the present invention. Said
medium is composed of a translucent substrate 1 provided with guide
grooves in advance (called "pregrooved"), and a a first magnetic
layer 2 and a second magnetic layer 3 laminated thereon. The first
magnetic layer 2 has a lower Curie point (T.sub.L) and a higher
coercive force (H.sub.H), while the second magnetic layer 3 has a
higher Curie point (T.sub.H) and a lower coercive force (H.sub.L).
The terms "higher" and "lower" are defined through relative
comparison of the two magnetic layers, the comparison of the
coercive force being at the room temperature. This comparison is
more detailedly illustrated in FIG. 4. It is generally desirable
that the first magnetic layer 2 has T.sub.L in a range of
70.degree.-180.degree. C. and H.sub.H in a range of 3-10 KOe, and
the second magnetic layer 3 has T.sub.H in a range of
150.degree.-400.degree. C. and H.sub.L in a range of 0.5-2 KOe.
The thickness, coercive force, saturated magnetization and magnetic
wall energy of said magnetic layers 2, 3 are so selected that the
two states of the finally recorded bit can exist in stable manner.
The magnetic layers 2, 3 may be exchange-coupled or
magnetostatically coupled although exchange-coupling is preferable
in consideration of the magnitude of the effective bias magnetic
field at the recording and the stability of the recorded binary
bit.
In the magnetooptical recording medium of the present invention,
the first magnetic layer 2 is principally related to the
reproduction. The magnetooptical effect exhibited by said first
magnetic layer 2 is principally utilized in the reproduction, while
the second magnetic layer 3 plays an important role in the
recording.
On the other hand, in the conventional exchange-coupled
double-layered magnetic film mentioned above, the magnetic layer
with a lower Curie point and a higher coercive force is principally
related to the recording, and the magnetic layer with a higher
Curie point and a lower coercive force is principally related to
the reproduction. In such conventional exchange-coupled
double-layered film, there stands a relationship: ##EQU2## among
the saturation magnetization M.sub.s of the latter magnetic layer,
film thickness h and magnetic wall energy .sigma..sub.w between two
layers.
On the other hand, in the exchange-coupled double-layered film of
the recording medium of the present invention, there is required a
relation: ##EQU3## among the saturation magnetization M.sub.s of
the second magnetic layer, film thickness h and magnetic wall
energy .sigma..sub.w between two layers.
This condition (detailed later) is required for stabilizing the
state of magnetization of the bit finally formed by recording, as
shown by 4f in FIG. 5. Consequently the effective bias magnetic
field, thickness, coercive force, saturation magnetization,
magnetic wall energy etc. of the magnetic layers 2, 3 can be so
determined as to satisfy the above-mentioned relation.
Each magnetic layer can be composed of a substance exhibiting a
vertical magnetic anisotropy and a magnetooptical effect,
preferably an amorphous magnetic alloy of a rare-earth element and
a transition metal element such as GdCo, GdFe, TbFe, DyFe, GdTbFe,
TbDyFe, GdFeCo, TbFeCo, GdTbCo or GdTbFeCo.
The following explanation of a recording process utilizing the
above-explained magnetooptical recording medium, makes reference to
FIG. 5, shows the states of magnetization of the magnetic layers 2,
3 in the steps of the recording process, while FIG. 6 schematically
shows a recording apparatus. Prior to recording, the stable
directions of magnetization of the magnetic layers 2, 3 may be
mutually same or opposite. FIG. 5 shows a case in which said stable
directions of magnetization are mutually same.
In FIG. 6, it is assumed that a part of a magnetooptical disk 9 of
the above-explained structure has an initial magnetization as shown
by 4a in FIG. 5. The magnetooptical disk 9, being rotated by a
spindle motor, passes the position of a magnetic field generating
unit 8, generating a magnetic field of which intensity is selected
at a suitable value between the coercive forces of the magnetic
layers 2, 3 (magnetic field being directed upwards in the present
embodiment), whereby, as shown by 4b in FIG. 5, the second magnetic
layer 3 is uniformly magnetized while the first magnetic layer 2
retains the initial magnetization state.
The rotated magnetooptical disk 9, in passing the position of a
record/reproducing head 5, is irradiated by a laser beam having two
power levels according to the signal from a recording signal
generator 6. The first laser power level is enough for heating the
disk to a temperature close to the Curie point of the first
magnetic layer 2, while the second laser power level is enough for
heating the disk to a temperature close to the Curie point of the
second magnetic layer 3. More specifically, referring to FIG. 4
showing the relation between the temperature and the coercive
forces of the magnetic layers 2, 3, the first laser power can heat
the disk close to T.sub.L while the second laser power can heat it
close to T.sub.H.
The first laser power heats the first magnetic layer 2 close to the
Curie point thereof, but the second magnetic layer 3 has a coercive
force capable of stably maintaining the bit at this temperature.
Thus, through a suitable selection of the recording bias magnetic
field, a record bit shown in 4c can be obtained, as a first
preliminary recording, from either state in 4b in FIG. 5.
The suitable selection of the bias magnetic field means that in the
first preliminary recording, such bias magnetic field is
essentially unnecessary, since the first magnetic layer receives a
force (exchange force) to arrange the magnetization in a direction
stable to the direction of magnetization of the second magnetic
layer 3, said directions being same in this case. However, said
bias magnetic field is provided, in a second preliminary recording
to be explained later, in a direction to assist the magnetic
inversion of the second magnetic layer 3, namely in a direction
prohibiting the first preliminary recording, and it is convenient
to maintain said bias magnetic field in a same intensity and a same
direction, both in the first and second preliminary recordings.
In consideration of the foregoing, the bias magnetic field is
preferably selected at a minimum necessary intensity required for
the second preliminary recording to be explained in the following,
and such selection corresponds to the suitable selection mentioned
above.
In the following there will be explained the second preliminary
recording. This is achieved by heating the disk with the second
laser power close to the Curie point of the second magnetic layer
3, whereby the direction of magnetization of the second magnetic
layer 3 is inverted by the bias magnetic field selected as
explained above, and the direction of magnetization of the first
magnetic layer 2 is also arranged in a stable direction (same
direction in the present case) with respect to the second magnetic
layer 3. In this manner a bit as shown by 4d in FIG. 5 can be
formed from either state shown in 4b.
Thus each area of the magnetooptical disk can have a preliminary
record of the state 4c or 4d in FIG. 5, respectively by the first
or second laser power corresponding to the input signal.
Then the magnetooptical disk 9 is further rotated and passes the
position of the magnetic field generating unit 8, generating a
magnetic field of which intensity is selected between the coercive
forces of the magnetic layers 2, 3 as explained before, whereby the
record bit 4c remains unchanged and assumes a final record state
4e, while the record bit 4d assumes another final record state 4f
as the result of magnetic inversion of the second magnetic layer
3.
In order that the record 4f can stably exist, there is required the
aforementioned relationship (1): ##EQU4## among the saturation
magnetization M.sub.s of the second magnetic layer 3, film
thickness h and magnetic wall energy .sigma..sub.w between the
magnetic layers 2, 3. .sigma..sub.w /2M.sub.s h indicates the
magnitude of the exchange force received by the second magnetic
layer, or represents the magnitude of a magnetic field acting to
rearrange the magnetization of the second magnetic layer 3 in a
stable direction (same direction in the present case) with respect
to the direction of magnetization of the first magnetic layer 2.
Therefore, in order that the second magnetic layer 3 can retain its
magnetization unchanged against said magnetic field, said layer
should have a coercive force H.sub.L larger than the magnitude of
said magnetic field (H.sub.L >.sigma..sub.w /2M.sub.s h).
Stated differently, in order that the bit can stably exist,
following relations are required among the coercive forces H.sub.H,
H.sub.L of the first and second magnetic layers and the effective
bias magnetic fields H.sub.Heff, H.sub.Leff of said layers:
These relations will be explained in more detail in relation to
FIGS. 7A to 7D. FIG. 7A is a chart showing the B-H loop, or the
relation between the external magnetic field, in abscissa, applied
to the first magnetic layer formed as a single layer, and the
magnitude of the magnetization in said layer in ordinate. The chart
indicates that, when the magnetic field is intensified in a
direction of (+), the magnetization is aligned in a direction (+)
or a direction (.uparw.) at an intensity H.sub.H, and, when the
magnetic field is intensified in a direction (-), the magnetization
is aligned in a direction (-) or (.dwnarw.) at an intensity
-H.sub.H. FIG. 7B shows a similar B-H loop for the second magnetic
layer formed as a single layer.
FIG. 7C shows a B-H loop of the first magnetic layer when the first
and second magnetic layers are superposed with exchange-coupling
and when said second magnetic layer is magnetized upward. In
contrast to the case of single layer shown in FIG. 7A, an effective
bias magnetic field H.sub.Heff is applied, facilitating to align
the magnetization of the first magnetic layer with that of the
second magnetic layer.
In order that the record bit 4f shown in FIG. 5 can stably exist,
the state of a point A, where the direction of magnetization of the
first magnetic layer is opposite to that of the second magnetic
layer under a zero external field should be stable and should not
transform to the state of a point B wherein said directions of
magnetization of the magnetic layers are mutually same. For this
reason there is required a condition H.sub.H -H.sub.Heff >0.
FIG. 7D shows a similar B-H loop of the second magnetic layer when
the first and second magnetic layers are superposed with
exchange-coupling and when said first magnetic layer is magnetized
upwards.
In contrast to the case of single layer shown in FIG. 7B, an
effective bias magnetic field H.sub.Leff is applied, facilitating
to align the magnetization of the second magnetic layer with that
of the first magnetic layer. In order that the record bit 4f in
FIG. 5 can stably exist, the state of a point A, where the
direction of magnetization of the first magnetic field is opposite
to that of the second magnetic field under a zero external field
should be stable and should not transform to the state of a point B
wherein said directions are mutually same. For this reason there is
required a condition H.sub.L -H.sub.Leff >0.
Either in the first or second magnetic layer, an inversion of
magnetization from a stable state to an unstable state requires a
magnetic field equal to the coercive force of the magnetic layer
plus the exchange force, since such inversion has to be made
against the exchange force acting on said layer.
On the other hand, an inversion from an unstable state to a stable
state requires a magnetic field equal to the coercive force of the
magnetic layer minus the exchange force, since the exchange force
facilitates the inversion in this case.
Therefore, in order that the magnetization of the first magnetic
layer is not inverted in the magnetic field generating unit 8 and
the magnetization of the second magnetic layer is aligned to the
direction of the magnetic field of said unit in any combination of
the magnetized states, the external field B should be adjusted to
an internal level if there stands a relation:
This is because the magnetic field required for inverting the
magnetization of the second magnetic layer is larger than H.sub.L
+H.sub.Leff when the first and second magnetic layers are in a
stable state, and because the magnetic field not inducing the
inversion of magnetization of the first magnetic layer should be
smaller than H.sub.H -H.sub.Heff when the first and second magnetic
layers are in a stable state.
An overwriting operation is therefore rendered possible, since the
record bits 4e, 4f do not rely on the state prior to recording but
only on the level of laser power at the recording. The record bits
4e, 4f can be reproduced by irradiation with a reproducing laser
beam and processing with a signal reproducing unit 7.
FIG. 8 is a schematic view showing a more detailed embodiment of
the recording apparatus shown in FIG. 6. In FIG. 8 there are shown
a magnetooptical disk or recording medium 11 of a structure as
shown in FIG. 3; a spindle motor 12 for rotating said disk 11; a
clamper 13 for fixing the disk 11 on the rotating shaft of the
motor 12; and an optical head 14 for projecting a light beam 15
onto the disk 11. Said optical head 14 is provided with a laser
light source 16 composed for example of a semiconductor laser; a
collimating lens 17; a beam splitter 18; an objective lens 19; a
sensor lens 20; an analyzer 26 and a photodetector 21, and is
radially moved by an unrepresented mechanism. Also the objective
lens 19 moves in the axial direction and a direction perpendicular
thereto to achieve so-called auto tracking (AT) and auto focusing
(AF), according to control signals detected by the photodetector in
an already known manner. The laser light source 16 is driven by a
laser driver circuit 22 and emits a light beam 15 modulated in
intensity between two non-zero values, according to the recording
information entered from an input terminal 23, as will be explained
later.
In a position opposed to the optical head 14 across the disk 11,
there is provided first magnetic field generating means 24 to apply
a bias magnetic field of a predetermined direction to an area of
the disk 11 irradiated by the light beam 15. Also at a position
distance by 180.degree. in the rotating direction of the disk 11,
there is provided second magnetic field generating means 25 for
applying a bias magnetic field of a direction opposite to said
predetermined direction. Said first and second magnetic field
generating means may be composed of electromagnets, but the use of
permanent magnets is preferable for simplifying the apparatus and
reducing the cost thereof, since the direction of magnetic fields
need not be switched in the present invention.
In the following explained is the process of information recording
with the apparatus shown in FIG. 8. The light beam 15 from the
laser unit 16 is modulated, as shown in FIG. 9, between two
non-zero levels P1 and P2, corresponding to binary recording
signals "0" and "1". The light beam of the level P1 has an energy
for heating the magnetic layers 2, 3 of the medium shown in FIG. 3
to the Curie temperature T.sub.L of the first magnetic layer, while
that of the level P2 has an energy for heating said magnetic layers
to the Curie temperature T.sub.H of the second magnetic layer. By
the irradiation with such modulated beam, the magnetic layers of
the disk 11 undergo changes in magnetization as shown in 10a-10h in
FIG. 10A, thereby recording information.
In FIG. 10A, arrows in the first and second magnetic layers 2, 3
indicate the directions of magnetization, and the length of arrow
indicates the magnitude of coercive force. An upward magnetized
area, shown in 10a, is subjected to an upward bias magnetic field
of -B2 by the second magnetic field generating means 25 as shown in
10b, and is then moved to the position of the optical head 14 by
the rotation of the disk for irradiation with the modulated light
beam 15. In response to the light beam of the power P1, the first
magnetic layer 2 alone reduces its coercive force as shown in 10c
but retains the upward magnetization as shown in 10d due to the
magnetic interaction of the second magnetic layer 3, despite of the
application of a bias magnetic field B1 by the first magnetic field
generating means 24. On the other hand, in response to the light
beam of the power P2, the magnetization is inverted downwards as
shown in 10h by the application of the bias field B1.
On the other hand, in a downward magnetized area as shown in 10e,
the second magnetic layer 3 alone inverts the magnetization by the
application of a bias magnetic field -B2. Then, in response to the
light beam of the level P1, the first magnetic layer 2 reduces the
coercive force of the first magnetic layer 2 as shown in 10c, and
inverts the magnetization upwards as shown in 10d by the magnetic
interaction of the second magnetic layer 3. Also in response to the
light beam of the power P2, both magnetic layer reduce the coercive
force as shown in 10g, and retain the downward magnetization as
shown in 10h by the application of the bias magnetic field B1.
As explained in the foregoing, the apparatus shown in FIG. 8 can
determine the direction of magnetization solely according to the
change in the power of the light beam, regardless of the initial
magnetization of the magnetic film. Consequently the already
recorded information need not be erased but can be rewritten by
direct overwriting. The recorded information can be reproduced in
conventional manner, by detecting the direction of magnetization of
the first magnetic layer utilizing the magnetooptical effect. As an
example, in the apparatus shown in FIG. 8, the recorded information
can be reproduced as electrical signals, by causing the laser unit
16 to continuously emit a light beam of a power insufficient for
heating the disk to the Curie point of the second magnetic layer
and receiving the reflected light from the disk 11 by the
photodetector 21 through the analyzer 26.
In the apparatus shown in FIG. 8, the magnetic fields B1, B2
applied to the magnetic layers respectively by the first and second
magnetic field generating means are so selected as to satisfy the
following relations:
wherein H.sub.H ' and H.sub.L ' are coercive forces of the first
and second magnetic layers at a temperature T.sub.L, H.sub.H " and
H.sub.L " are coercive forces at a temperature T.sub.H, and H.sub.o
is the magnitude of the magnetic interaction between two
layers.
In the foregoing description it is assumed the first and second
magnetic layers 2, 3 are stable when the magnetizations thereof are
in a same direction, but a similar process is applicable also when
said magnetizations are stable when they are mutually oppositely
directed. FIG. 10B illustrates states of magnetization in the
recording process of such case, wherein 40a to 40f respectively
correspond to 4a to 4f in FIG. 5.
EXAMPLE 1
A polycarbonate substrate with pregrooves and preformat signals was
set in a sputtering apparatus with three targets, and was rotated
at a distance of 10 cm from the target.
A ZnS protective layer of 1000 .ANG. in thickness was obtained by
sputtering from a first target in argon gas, with a sputtering
speed of 100 .ANG./min., and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered from a
second target, in argon gas, with a sputtering speed of
100.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr
to obtain a first magnetic layer of Tb.sub.18 Fe.sub.82 with a
thickness of 500 .ANG., T.sub.L of about 140.degree. C. and H.sub.H
of about 10 KOe.
Then a TbFeCo alloy was sputtered in argon gas at a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic
layer of Tb.sub.23 Fe.sub.60 Co.sub.17 with a thickness of 500
.ANG., T.sub.H of ca. 250.degree. C. and H.sub.L of ca. 1 KOe.
Subsequently a ZnS protective layer of 3000 .ANG. in thickness was
obtained by sputtering from the first target in argon gas, with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was
adhered to a polycarbonate substrate with hot-melt adhesive
material to obtain a magnetooptical disk. Said disk wa mounted on a
record/reproducing apparatus and was made to pass through, with a
linear speed of 8 m/sec., the second magnetic field generating
means for applying a field of 1500 Oe to the disk. The recording
was conducted with a laser beam of a wavelength of 830 nm,
concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8
mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias
magnetic field was 100 Oe. Binary signals could be reproduced by
irradiation with a laser beam of 1.5 mW.
The above-explained experiment was conducted on a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
and the possibility of overwriting was thus confirmed.
EXAMPLE 2
A magnetooptical disk was prepared in the same manner as in the
Example 1, except that the second magnetic layer was composed of
Tb.sub.23 Fe.sub.70 Co.sub.7 with T.sub.H =200.degree. C., H.sub.L
=ca. 1 KOe and H.sub.Leff =ca. 300 Oe, and was subjected to
recording and reproduction in the same manner as in the Example 1
except the use of a magnetic field generating field of ca. 2.5 KOe
to obtain similar results as those in the Example 1.
The magnitude of H.sub.Heff in this case will be explained in the
following in relation to FIGS. 7C and 7D.
Since H.sub.H -H.sub.Heff was larger than H.sub.L -H.sub.Leff, when
the first and second magnetic layers were magnetized in a same
direction and subjected then to an inverting magnetic field, the
magnetization of the second magnetic layer was inverted at H.sub.L
-H.sub.Leff =0.7 KOe so that H.sub.Leff could not be measured.
However, based on the condition H.sub.H -H.sub.Heff >H.sub.L
-H.sub.Leff, a conclusion 4.3 KOe>H.sub.Heff from the conditions
H.sub.H =5 KOe, H.sub.L =1 KOe and H.sub.Leff =0.3 KOe.
Also the H.sub.Heff was measured as ca. 1 KOe in an experiment in
which a first magnetic TbFe layer of same composition and thickness
was superposed with a second magnetic TbFeCo layer with a modified
composition to increase H.sub.L -H.sub.Leff.
It was confirmed, for the above-mentioned coercive forces and
exchange force, that the first and second magnetic layers satisfied
the condition:
by which the second magnetic layer alone is magnetized in the
direction of the magnetic field from the magnetic field generating
unit 24.
EXAMPLE 3
A polycarbonate disk-shaped substrate with pregrooves and preformat
signals was set in a sputtering apparatus with three targets, and
was rotated at a distance of 10 cm from the target.
A protective SiC layer of 700 .ANG. in thickness was obtained by
sputtering from a first target in argon gas, with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
3.times.10.sup.-3 Torr. Then a GdTbFe alloy was sputtered from a
second target, in argon gas, with a sputtering speed of 50
.ANG./min. and a sputtering pressure of 3.times.10.sup.-3 Torr to
obtain a first magnetic layer of Tb.sub.8 Gd.sub.12 Fe.sub.80 with
a thickness 200 .ANG., T.sub.L =ca. 160.degree. C. and H.sub.H =ca.
8 KOe.
Then a TbFeCo Cu alloy was sputtered in argon gas at a sputtering
pressure of 3.times.10.sup.-3 Torr to obtain a second magnetic
layer of Tb.sub.23 Fe.sub.50 Co.sub.15 Cu.sub.12 with a thickness
of 400 .ANG., T.sub.H =ca. 180.degree. C. and H.sub.L =ca. 1
KOe.
Subsequently a Si.sub.3 N.sub.4 protective layer of 1200 .ANG. in
thickness was obtained by sputtering from the first target in argon
gas, with a sputtering speed of 70 .ANG./min. and a sputtering
pressure of 3.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. Said disk was mounted on a
record/reproducing apparatus and was made to pass through, a with a
linear speed of 8 m/sec., the second magnetic field generating
means for applying a field of 1500 Oe to the disk. The recording
was then conducted with a laser beam of a wavelength of 830 nm,
concentrated to ca. 1 .mu.m and modulated in two levels of 4 and 8
mW, with a duty ratio of 50% and a frequency of 2 MHz. The bias
field at the irradiated area was 100 Oe. Binary signals could be
reproduced by irradiation with a laser beam of 1.5 mW.
The above-explained experiment was conducted on a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
and the possibility of overwriting was thus confirmed.
In case the apparatus shown in FIG. 6 employs a permanent magnet as
the magnetic field generating unit 8, the magnetooptical recording
medium of the present invention is constantly exposed, in the
recording and reproducing operations, to a magnetic field generated
by said magnetic field generating unit. Even in the reproduction,
the medium is subjected to a laser beam irradiation of an energy of
ca. 1/3 to 1/10 of the energy at the recording, and may therefore
reach a temperature of about 70.degree. C. at maximum in passing
the magnetic field generating unit. Thus the magnetization of the
first magnetic layer 2 may become inverted in repeated
reproductions, for example of 10.sup.10 times. In order to avoid
such inversion of magnetization, it is preferable, as shown in the
following examples, to select the coercive force H.sub.H of the
first magnetic layer and the magnetic field B applied by the
magnetic field generating unit in such a manner as to satisfy a
relation 0.2.times.H.sub.H -0.3>B (KOe), and, more prefearbly a
condition H.sub.H >1.5 KOe. A further preferred condition is
H.sub.H >5 KOe as will be explained in the following.
EXAMPLE 4
Samples of magnetooptical disks 4-1 to 4-12 were prepared in the
same process as in the Example 1, with the same film thickness and
the same structure except that the composition, coercive force
H.sub.H and Curie point T.sub.L of the first magnetic layer 2 were
changed.
Each disk of Examples 1 and 4-1 to 4-12 was mounted on a
record/reproducing apparatus and was made to pass through, with a
linear speed of 8 m/sec., the second magnetic field generating
means for applying a field of 2 KOe. The recording was conducted
with a laser beam of a wavelength of 830 nm, concentrated to ca. 1
.mu.m and modulated in two levels of 4 and 8 mW, with a duty ratio
of 50% and a frequency of 2 MHz. The bias magnetic field at the
recording head was 100 Oe. Then investigated was the change of the
noise component in the reproduced signal, after 10.sup.10
reproductions from a same track by irradiation with a laser beam of
1.0 mW and with varied magnetic field B generated by the magnetic
field generating unit 8.
Subsequently the temperature inside the apparatus was set at
30.degree., 45.degree. and 60.degree. C. and the intensity of
magnetic field causing the increase of noise component in the
reproduced signal was determined for each temperature, as
summarized in Tab. 1.
TABLE 1
__________________________________________________________________________
B value (KOe) B value (KOe) B value (KOe) causing noise causing
noise causing noise lst mag. H.sub.H T.sub.L increase at increase
at increase at Example layer (KOe) (.degree.C.) 30.degree. C.
45.degree. C. 60.degree. C.
__________________________________________________________________________
1 Tb.sub.18 Fe.sub.82 10 140 8 6 3.2 4-1 Tb.sub.17 Fe.sub.83 8 140
6 4.7 2.6 4-2 Tb.sub.16 Fe.sub.84 6 140 4 3.1 1.5 4-3 Tb.sub.15
Fe.sub.85 4 140 2.2 1.5 0.5 4-4 Tb.sub.14.5 Fe.sub.85.5 3 140 1.5
1.0 0.3 4-5 Tb.sub.14 Fe.sub.86 2 140 1.2 0.5 0.1 4-6 Tb.sub.10
Gd.sub.7 Fe.sub.83 7 150 4.8 3.7 2.2 4-7 Tb.sub.10 Gd.sub.6
Fe.sub.84 5 150 3.3 2.3 1.0 4-8 Tb.sub.10 Gd.sub.5 Fe.sub.85 3.5
150 2.8 1.2 0.4 4-9 Tb.sub.10 Gd.sub.7 Fe.sub.80 Co.sub.3 7.5 165
5.4 4.3 2.5 4-10 Tb.sub.10 Gd.sub.7 Fe.sub.80 Co.sub.4 5.5 170 3.5
2.7 1.3 4-11 Tb.sub.15 Fe.sub.81 Co.sub. 4 4.5 160 2.5 1.8 0.6 4-12
Tb.sub.14.5 Fe.sub.82 Co.sub.3.5 3.5 160 1.8 1.2 0.4
__________________________________________________________________________
Tab. 1 indicates that the magnetic field B inducing the increase of
noise component in the reproduced signal becomes smaller as the
coercive force H.sub.H of the first magnetic layer becomes smaller,
or as the temperature inside the apparatus becomes higher, and that
this relationship is not affected by the composition of the first
magnetic layer.
FIG. 11 illustrates the relation of H.sub.H value and the B value
inducing the noise increase for these samples, based on the
above-explained results.
From these relationships it is apparent that, in order to prevent
the noise increase up to 60.degree. C., H.sub.H and B should at
least satisfy a relation: 0.2.times.H.sub.H -0.3>B.
It is also apparent that the increase in noise takes placed even at
a small value of B unless H.sub.H is at least equal to 1.5 KOe.
Since the temperature inside the apparatus does not exceed
60.degree. C. in practice, the increase in noise can be prevented
even after prolonged reproduction if the above-mentioned conditions
are satisfied.
However, in addition to the foregoing conditions, it is preferable
to maintain H.sub.H at least equal to 5 KOe for the following two
reasons:
(i) The minimum necessary value of B is equal to the sum of the
coercive force H.sub.L of the second magnetic layer and the
magnetic field acting on the second magnetic layer as the result of
the exchange force, and is considered in the order of 1 to 2 KOe in
practice. Thus, it will be apparent from FIG. 11 that the value of
H.sub.H should be equal to or higher than 5 KOe in order to
maintain the noises at a low level in continuous reproduction at a
temperature up to 60.degree. C. and with the magnetic field B at
1-2 KOe.
(ii) FIG. 11 indicates the magnitude of the magnetic field B and
the value of H.sub.H required for suppressing the noise at the
selected value of B at each temperature inside the apparatus. For
example, at a temperature 60.degree. C. and in a range H.sub.H
<5 KOe, there is obtained a relation H.sub.H =5 (B+0.3). Thus,
for an increase .DELTA.B of the magnetic field B generated by the
magnetic field generating unit, there is required a corresponding
increase 5.times..DELTA.B in the value of H.sub.H.
However, in a range of H.sub.H exceeding 5 KOe, the increase
required for H.sub.H corresponding to a certain increase in the
magnetic field B at a temperature of 30.degree.-60.degree. C. is
smaller than that required in a range of H.sub.H below 5 KOe.
As explained in the foregoing, the magnetooptical recording medium
of the present invention is required to satisfy the aforementioned
relation (1): ##EQU5## For this purpose it is also very effective
to adjust the magnetic wall energy between the magnetic layers. The
adjustment of the magnetic wall energy can be achieved by following
methods:
(I) adjustments of the composition of the magnetic layers;
(II) addition of a predetermined step in the preparation of the
medium; and
(III) formation of an adjusting layer for the magnetic wall energy,
between the magnetic layers;
which will be explained further in the following.
I. Adjustment of the composition of the magnetic layers
In a magnetooptical recording medium of the structure as shown in
FIG. 3, the magnetic wall energy can be reproducibly reduced by
forming one of the first and second magnetic layer 2, 3 with a
composition richer in transition metals compared with the
compensation composition, and forming the other layer with a
composition richer in rare-earth elements. Such example is shown in
the following.
EXAMPLE 5
A disk-shaped polycarbonate substrate with pregrooves and preformat
signals was set in a sputtering apparatus with three targets, and
was rotated at a distance of 10 cm from the target.
A protective Si.sub.3 N.sub.4 layer of 600.ANG. in thickness was
obtained by sputtering from a first target in argon gas, with a
sputtering speed of 40.ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered from a
second target, in argon gas, with a sputtering speed of
100.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr
to obtain a first magnetic layer of Tb.sub.3 Gd.sub.16 Fe.sub.81,
richer in Fe with respect to the compensation composition, with a
thickness of 400 .ANG., T.sub.L =ca. 155.degree. C. and H.sub.H
=ca. 8 KOe.
Then a TbFeCo alloy was sputtered in argon at a sputtering pressure
of 5.times.10.sup.-3 Torr to obtain a second magnetic layer of
Tb.sub.10 Dy.sub.13 Fe.sub.60 Co.sub.17, richer in Tb and Dy with
respect to the compensation composition, with a thickness of 300
.ANG., T.sub.H =ca. 200.degree. C. and H.sub.L =ca. 1 KOe.
Subsequently a Si.sub.3 N.sub.4 protective layer of 1500 .ANG. in
thickness was obtained by sputtering from the first target in argon
gas, with a sputtering speed of 40 .ANG./min. and a sputtering
pressure of 5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. Said disk was mounted on a
record/reproducing apparatus and was made to pass through, with a
linear speed of 8 m/sec., a magnetic field generating unit for
applying a field of 2.5 KOe. The recording was conducted with a
laser beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m
and modulated in two levels of 4 and 8 mW, with a duty ratio of 50%
and a frequency of 2 MHz. The bias magnetic field at the irradiated
area was 100 Oe, in a direction to invert the magnetization of the
second magnetic layer. Binary signals could be reproduced by
irradiation with a laser beam of 1.5 mW.
The above-explained experiment was conducted also on a
magnetooptical disk already recorded over the entire surface. The
previously recorded signal components were not detected in the
reproduction, and the possibility of overwriting was thus
confirmed.
EXAMPLE 6
In the Example 5, the first magnetic layer was richer in Fe while
the second magnetic layer was richer in Tb and Dy in comparison
with the compensation composition.
In the present example, there were prepared and evaluated samples
of the magnetooptical disk, in which the first and second magnetic
layers had same coercive forces as explained above, are composed of
a combination of a composition richer in transition metals such as
Fe and a composition richer in rare earth elements such as Tb or
Dy.
A coercive force of 8 KOe for the first magnetic layer is achieved,
in a transition metal-rich composition, by Tb.sub.3 Gd.sub.16
Fe.sub.81, or, in a coercive composition, by Tb.sub.3.3 Gd.sub.17.7
Fe.sub.77. Also a coercive force of 1 KOe for the second magnetic
layer is achieved, in a transition metal-rich composition by
Tb.sub.7.4 Dy.sub.9.6 Fe.sub.64.7 Co.sub.18.3, or, in a rare
earth-rich composition, by Tb.sub.10 Dy.sub.13 Fe.sub.60
Co.sub.17.
Samples listed in Tab. 2 were prepared by selecting a transition
metal-rich composition or a rare earth-rich composition mentioned
above for the first and second magnetic layer, and selecting other
materials and layer thicknesses same as those in the Example 5.
These samples were then subjected to the record/reproducing
experiment as in the Example 5. The results are shown in Tab.
2.
"TM" and "RE" respectively show a composition richer in the
transition metals and a composition richer in the rare earth
elements, compared to the compensation composition.
TABLE 2
__________________________________________________________________________
Evaluation of record bits 1st mag. layer 2nd mag. layer bit 4e in
bit 4f in Example (TM or RE rich) (TM or RE rich) FIG. 5 FIG. 5
__________________________________________________________________________
5 Tb.sub.3 Gd.sub.16 Fe.sub.81 (TM) Tb.sub.10 Dy.sub.13 Fe.sub.60
Co.sub.17 +RE) + 6-1 Tb.sub.3.3 Gd.sub.17.7 Fe.sub.79 (RE)
Tb.sub.7.4 Dy.sub.9.6 Fe.sub.64.7 Co.sub.18.3 +TM) + 6-2 Tb.sub.3
Gd.sub.16 Fe.sub.81 (TM) Tb.sub.7.4 Dy.sub.9.6 Fe.sub.64.7
Co.sub.18.3 -TM) .+-. 6-3 Tb.sub.3.3 Gd.sub.17.7 Fe.sub.79 (RE)
Tb.sub.10 Dy.sub.13 Fe.sub.60 Co.sub.17 -RE) .+-.
__________________________________________________________________________
"+" indicates that the record bits are stable in the absence of
external magnetic field and provide satisfactory reproduction
signals; ".+-." indicates that the record bits are partially
inverted or the reproduction signals are of insufficient quality;
and "-" indicates that the record bits are unstable.
These results indicate that stable record bits are obtained only
when one of the first and second magnetic layers is composed of the
transition metal-rich composition and the other is composed of the
rare earth-rich composition.
II. Addition of a predetermined step in the preparation of the
medium
In the preparation of the magnetooptical recording medium as shown
in FIG. 3, a medium satisfying the afore-mentioned relation (1) can
be easily obtained by adding one of following steps after the
formation of the first magnetic layer and before the formation of
the second magnetic layer:
(A) a step of standing in an atmosphere of remaining gas or inert
gas at 7.times.10.sup.-7 Torr for 5 minutes of longer;
(B) a step of standing in an atmosphere with a partial pressure, at
least equal to 2.times.10.sup.-6 Torr, of a substance capable of
reacting with a constituent element of the first or second magnetic
layer or being chemically absorbed by said element; or
(C) a step of exposure to a plasma atmosphere of inert gas or a
substance capable of reacting with a constituent element of the
first or second magnetic layer or being chemically absorbed by said
element.
The first and second magnetic layers can be formed by sputtering,
or evaporation for example with electron beam heating.
Examples of the above-mentioned remaining gas are H.sub.2 O,
O.sub.2, H.sub.2, N.sub.2 and low-molecular compounds consisting of
C, H, N and O, and examples of the inert gas are Ar, He and Ne.
Examples of the gas capable of reacting with the constituent
element of the first or second magnetic layer or being chemically
absorbed by said element are H.sub.2 O, O.sub.2, H.sub.2, N.sub.2,
H.sub.2 S, CS.sub.2 and CH.sub.4.
In the usual manufacturing process for the medium, after the
formation of the first magnetic layer, the formation of the second
magnetic layer is conducted immediately (for example within 1
minute) in a clean high-vacuum atmosphere. However the addition of
one of the steps (A)-(C) modifies the exchange force, coercive
force or stability of the magnetic layers, and a reproducible
recording characteristic can be obtained through precise control of
the process conditions, process time etc.
The effects of the steps (A)-(C) will be verified in the following
examples.
EXAMPLE 7
A pregrooved and proformatted polycarbonate disk substrate was set
in a sputtering apparatus with three targets, and was rotated at a
distance of 10 cm from the target.
The apparatus was evacuated to 1.times.10.sup.-7 Torr, and a
protective SiO layer of 1000 .ANG. in thickness was obtained by
sputtering from a first target in argon gas, with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered from a
second target, in argon gas, with a sputtering speed of 100
.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to
obtain a first magnetic layer composed of Tb.sub.18 Fe.sub.82 with
a thickness of 300.ANG., T.sub.L =ca. 140.degree. C. and H.sub.H
=ca. 10 KOe. After the completion of sputtering, argon gas supply
was continued for 30 minutes, with a pressure of 5.times.10.sup.-3
Torr in the sputtering chamber.
Then a TbFeCo alloy was sputtered in argon gas with a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic
layer composed of Tb.sub.23 Fe.sub.70 Co.sub.7 with a thickness of
400.ANG., T.sub.H =ca. 200.degree. C. and H.sub.L =ca. 1 KOe.
Subsequently a SiO protective layer of 2000.ANG. in thickness was
formed by sputtering from the first target in argon gas, with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After forming these layer, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. Said disk was mounted on a
record/reproducing apparatus and was made to pass through, with a
linear speed of 8 m/sec., a unit for generating a magnetic field of
2.5 KOe. The recording was conducted with a laser beam of a
wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in
two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency
of 2 MHz. The bias magnetic field was 100 Oe. Binary signals could
be reproduced by irradiation of a laser beam of 1.5 mW.
The above-explained experiment was repeated also on a
magnetooptical disk already recorded over the entire surface. The
previously recorded signal components were not detected in the
reproduction, so that the possibility of overwriting was thus
comfirmed.
EXAMPLE 8 AND REFERENCE EXAMPLE
Samples of the magnetooptical disk were prepared in a process
similar to that of the Example 7 but with varied conditions
(atmosphere and pressure) in the step between the formations of the
first and second magnetic layers, as listed in Tab. 3. Those marked
with * are examples of the present invention, and others are
reference examples.
In examples 8-30 to 8-33, a disk electrode of 20 cm in diameter was
placed at 5 cm from the polycarbonate substrate, and a plasma
treatment was conducted with a discharge power of 50 W, in the
presence of various gasses listed in Tab. 3 at a pressure of
5.times.10.sup.-3 Torr in the sputtering chamber. In examples 8-5
to 8-14, the main valve of the vacuum pump was suitably closed to
vary the remaining gas atmosphere.
Each sample was evaluated for the stability of the record bits 4f,
shown in FIG. 5, in the absence of external magnetic field, through
the measurement of an external magnetic field inducing the
inversion of magnetization in the magnetic layers. "+" and "-"
respectively indicate that the record bits are stable or
unstable.
Also each samples was tested for recording and reproduction in the
same manner as in the Example 7. "+" and "-" respectively indicate
that the recording was satisfactorily or unsatisfactorily made.
TABLE 3 ______________________________________ Sample evaluation
Vacuum Time Stability Record Example Atmosphere (Torr) (min) of
bits 4f state ______________________________________ *Ex. 7 argon
gas 5 .times. 10.sup.-3 30 + + 8-1 " " 1/4 - - *8-2 " " 2 - - *8-3
" " 5 + + 8-4 " " 15 + + 8-5 remaining gas 1 .times. 10.sup.-6 1/4
- - 8-6 " " 2 - - *8-7 " " 5 + + *8-8 " " 15 + + *8-9 " " 30 + +
8-10 remaining gas 3 .times. 10.sup.-6 1/4 - - 8-11 " " 2 - - *8-12
" " 5 + + *8-13 " " 15 + + *8-14 " " 30 + + *8-15 oxygen gas 3
.times. 10.sup.-6 1/2 + + *8-16 " " 2 + + *8-17 " " 5 + + *8-18 " "
15 + + *8-19 " " 30 + + *8-20 nitrogen gas 3 .times. 10.sup.-6 1/2
+ + *8-21 " " 2 + + *8-22 " " 5 + + *8-23 " " 15 + + *8-24 nitrogen
gas 3 .times. 10.sup.-6 30 + + *8-25 hydrogen gas 3 .times.
10.sup.-6 1/2 - - *8-26 " " 2 + + *8-27 " " 5 + + *8-28 " " 15 + +
*8-29 " " 30 + + *8-30 argon plasma 3 .times. 10.sup.-3 1/12 + +
*8-31 oxygen plasma " 1/12 + + *8-32 nitrogen plasma " 1/12 + +
*8-33 hydrogen " 1/12 + + plasma
______________________________________
Results of the Example 7, Examples 8 and Reference Examples
indicate that the recording by overwriting can be satisfactorily
achieved by the magnetooptical recording medium prepared employing
either one of the steps (A) to (C).
III. Formation of a magnetic wall energy adjusting layer between
the magnetic layers
Formation of an adjusting layer 41, as shown in FIG. 12, between
the first and second magnetic layers 2, 3 allows to arbitrarily
regulate the magnetic wall energy therebetween, thereby obtaining a
medium satisfying the aforementioned condition (1). More
practically there may be employed a structure shown in FIG. 13,
using a pregrooved substrate 1 and provided with protective layers
42, 43. In FIGS. 12 and 13, same components as those in FIG. 3 are
represented by same numbers and will not be explained further.
Said adjusting layer 41 may be composed of a material not
deteriorating the magnetic layers, for example Ti, Cr, Al, Ni, Fe,
Co, rare earth element or a fluoride thereof.
The thickness of said adjusting layer 41 is suitably selected in
consideration of the materials and thicknesses of the first and
second magnetic layers, but is generally selected within a range of
5 to 50 .ANG..
EXAMPLE 9
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with ternary targets, and was rotated at
a distance of 10 cm from the target.
A ZnS protective layer of 1000 .ANG. in thickness was formed by
sputtering from a first target in argon gas, with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered from a
second target, in argon gas, with a sputtering speed of 100
.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to
obtain a first magnetic layer composed of Tb.sub.18 Fe.sub.82 with
a thickness of 500 .ANG., T.sub.L =ca. 140.degree. C. and H.sub.H
=ca. 5 KOe.
Then a Co adjusting layer was formed with a thickness of 10 .ANG.,
by sputtering in argon gas with a sputtering pressure of
5.times.10.sup.-3 Torr. Then TbFe and Co were simultaneously
sputtered from second and third targets in argon gas with a
sputtering pressure of 5.times.10.sup.-3 Torr to obtain a second
magnetic layer composed of Tb.sub.15 Fe.sub.68 Co.sub.17 with a
thickness of ca. 200 .ANG., T.sub.H =ca. 250.degree. C. and H.sub.L
=ca. 2 KOe.
Subsequently a ZnS protective layer of 3000 .ANG. in thickness was
formed by sputtering from the first target in argon gas, with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. Said disk was mounted on a
record/reproducing apparatus and was made to pass through, with a
linear speed of 8 m/sec., a unit for generating a magnetic field of
2.5 KOe. The recording was then conducted with a laser beam of a
wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in
two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency
of 2 MHz. The bias magnetic field was 100 Oe. Binary signals could
be reproduced by irradiation with a laser beam of 1.5 mW.
The above-explained experiment was repeated with a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
so that the possibility of overwriting was thus confirmed.
The adjusting layer was composed of Co in the foregoing embodiment,
but it may also be composed of a magnetic material of which easy
direction of magnetization is positioned longitudinally along the
disk surface at room temperature but vertically to the disk surface
at the recording temperature. The use of such material reduces the
magnetic wall energy between the magnetic layers at room
temperature and provides a larger exchange force to the magnetic
layers at recording, thereby providing a magnetooptical recording
medium enabling overwrite recording with a smaller bias magnetic
field and superior in the stability of the record bits. Such
structure will be shown in the following examples.
A sample for exchange force measurement was prepared by sputtering,
on a slide glass, a first magnetic layer of Tb.sub.18 Fe.sub.82 of
500 .ANG. in thickness, then an adjusting layer of Fe or Tb.sub.25
Fe.sub.70 Co.sub.5 in various thicknesses, and a second magnetic
layer of Tb.sub.22 Fe.sub.70 Co.sub.8 of 500 .ANG. in thickness.
The first magnetic layer showed a coercive force of 12 KOe, with
prevailing Fe sub-lattice magnetization, while the second magnetic
layer showed a coercive force of 6 KOe, with prevailing Tb
sub-lattice magnetization.
Each sample was subjected to the measurement of the external
magnetic field inducing the inversion of magnetization of the first
and second magnetic layers, in a VSM and in the presence of an
external magnetic field. In a decreasing magnetic field, the
samples showed an inversion of magnetization of the second magnetic
layer into a stable (opposite) direction with respect to that of
the first magnetic layer. The exchange force applied to the second
magnetic layer was determined from such inversion-inducing magnetic
field, as shown in FIG. 14, which indicates the exchange force on
the second magnetic layer in ordinate and the thickness of the
adjusting layer (Fe or TbFeCo) in abscissa.
As will be apparent from this chart, the exchange force is
annulated even at a layer thickness of 70 .ANG. with a Fe layer
having a longitudinal easy direction of magnetization. On the other
hand, in case of the Tb.sub.25 Fe.sub.70 Co.sub.5 with a coercive
force of ca. 300 Oe and with a vertical easy direction of
magnetization, parallel to those of the first and second magnetic
layer, the exchange force is still effective even at a layer
thickness of 500 .ANG..
Therefore, the stability of record bits and the stable recording
characteristic can be both obtained by forming, between the first
and second magnetic layers, an adjusting layer of a material of
which easy direction of magnetization is longitudinal at room
temperature but is vertical at the recording temperature.
Temperature-dependent change of easy direction of magnetization is
already known in substances showing spin rearrangement. For
example, DyCo.sub.5, reported by M. Ohkoshi and H. Kobayashi in
Physica, 86-88B (1977), p. 195-196, exhibits a change of the easy
direction of magnetization from longitudinal to vertical in a
temperature range of 50.degree.-100.degree. C. Similar results are
known in compounds in which Dy is replaced by another rare earth
element such as Nd, Pr or Tb, or in which Co is replaced by another
transition metal such as Fe or Ni. Also Tsushima reported, in Oyo
Buturi, 45, 10 (1976), p. 962-967, the spin rearrangement in rare
earth orthoferrites and rare earth orthochromites. A suitable
modification of the composition of these substances allows to
achieve a change of the easy direction of magnetization from
longitudinal to vertical state in the recording temperature
range.
Also it is already known that a thin magnetic film has to satisfy a
condition:
in order to have a magnetization vertical to the film surface,
wherein M.sub.s is the saturated magnetization and H.sub.k is the
uniaxial anisotropic magnetic field in said vertical direction.
Therefore, in order that the adjusting layer has the easy
magnetization axis in the longitudinal direction at room
temperature and in the vertical direction at the recording
temperature range, it is desirable to select the Curie point of
said adjusting layer in the vicinity of said recording temperature.
Since M.sub.s shows a rapid decrease in the vicinity of the Curie
point, a substance showing a relation H.sub.k <4.pi.M.sub.s at
room temperature may show a relation H.sub.k .gtoreq.4.pi.M.sub.s
in the recording temperature range. As the component vertical to
the substrate surface increases by the magnetization of the
adjusting layer, said magnetization is further oriented vertical to
the substrate surface by the exchange forces from the first and
second magnetic layers. The exchange forces H.sub.eff(1-2) and
H.sub.eff(2-3) working on the adjusting layer respectively from the
first and second magnetic layers can be represented by:
wherein M.sub.s2 is the saturation magnetization of the adjusting
layer, h.sub.2 is the thickness thereof, and .sigma..sub.w12
.sigma..sub.w23 are magnetic wall energies respectively between the
first magnetic layer and the adjusting layer and between the second
magnetic layer and the adjusting layer.
Therefore, in order to orient the magnetization of the adjusting
layer in a direction vertical to the film surface in the recording
temperature range by means of said exchange forces H.sub.eff(1-2)
and H.sub.eff(2-3), it is advantageous to select a small saturation
magnetization M.sub.s and a thickness h.sub.2, as long as the easy
direction of magnetization remains longitudinal at room
temperature.
EXAMPLE 10
A pregrooved and preformatted polycarbonate disk substrate was
placed in a sputtering apparatus with quaternary targets, and was
rotated at a distance of 10 cm from the targets.
A Si protective layer of 500 .ANG. in thickness was sputtered from
a first target, in argon gas, with a sputtering speed of 100
.ANG./min., and a sputtering pressure of 5.times.10.sup.-3 Torr.
Then a GdTbFe alloy was sputtered from a second target, in argon
gas, with a sputtering speed of 100 .ANG./min. and a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer
composed of Tb.sub.12 Gd.sub.10 Fe.sub.78 with a thickness of 300
.ANG., T.sub.L =ca. 150.degree. C. and H.sub.H =ca. 8 KOe. Fe was
prevailing in the sub-lattice magnetization of the first magnetic
layer.
Then a TbFeCo alloy was sputtered from a third target, in argon
gas, with a sputtering speed of 100 .ANG./min. and a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain an adjusting layer,
composed of Tb.sub.35 Fe.sub.60 Co.sub.5 having a thickness of 200
.ANG. and a coercive force of almost zero at the Curie temperature
of ca. 170.degree. C. The easy direction of magnetization of said
adjusting layer was neither longitudinal nor vertical, and the
external magnetic field required for orienting the magnetization in
either direction was ca. 2.5 KOe.
Then a TbFeCo alloy was sputtered from a fourth target, in argon
gas, with a sputtering speed of 100 .ANG./min and a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic
layer composed of Tb.sub.24 Fe.sub.68 Co.sub.8 with a thickness of
300 .ANG., T.sub.H =ca. 180.degree. C. and H.sub.L =ca. 1.5 KOe. Tb
was prevailing in the sub-lattice magnetization of said second
magnetic layer.
Subsequently a Si protective layer of 1000 .ANG. in thickness was
formed by sputtering from the first target, in argon gas, with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk.
The effective bias magnetic field, caused by the exchange force on
the second magnetic layer, was almost zero in thus prepared
magnetooptical disk, when measured in the same manner as in FIG.
14.
Said magnetooptical disk was mounted on a record/reproducing
apparatus and was made to pass through, with a linear speed of ca.
8 m/sec., a unit generating a magnetic field of 2.5 KOe. The
recording operation was then conducted with a laser beam of a
wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in
two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency
of 2 MHz. The bias magnetic field was 100 Oe. Binary signals could
be reproduced by irradiation with a laser beam of 1.5 mW.
The above-explained experiment was repeated with a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
so that the possibility of overwriting was confirmed.
EXAMPLE 11
Samples of the magnetooptical disk were prepared with the same
process and materials as in the Example 10, except that the
material and thickness of the adjusting layer were varied.
The prepared samples were subjected to the measurements of the
effective bias field caused by the exchange force on the second
magnetic layer and the recording characteristic in the same manner
as in the Example 10. The results are summarized in Tab. 4.
TABLE 4
__________________________________________________________________________
Effective bias Recording Adjusting Thickness of field on 2nd
character- Example layer adjusting la. layer istic
__________________________________________________________________________
10 Tb.sub.35 Fe.sub.60 Co.sub.5 200 .ANG. 0 Oe Good 11-1 None --
2000 Oe First-type record not possible 11-2 Fe.sub.70 Cr.sub.30 40
.ANG. 200 Oe Good 11-3 DyCo.sub.5 200 .ANG. 0 Oe Good 11-4
Sm.sub.0.7 Fe.sub.0.3 FeO.sub.3 150 .ANG. 300 Oe Good 11-5 Si 40
.ANG. 250 Oe First-type record sensitivity low, bit error rate high
11-6 Si 60 .ANG. 150 Oe First-type record not possible
__________________________________________________________________________
The sample 11-1 was not provided with the adjusting layer. In this
case stable record bits could not be formed because the effective
bias magnetic field, required for orienting the magnetization of
the second magnetic layer in a direction stable with respect to
that of the first magnetic layer, was larger than the coercive
force of the second magnetic layer.
The Fe.sub.70 Cr.sub.30 film employed in the sample 11-2 has a
Curie point lower than 200.degree. C. and a longitudinal easy axis
of magnetization. With a thickness not exceeding 30 .ANG., the
magnetization was oriented in the vertical direction by the
exchange forces from the first and second magnetic layers.
Therefore the effective bias field on the second magnetic layer
became larger and stable recording could not be achieved. On the
other hand, with a thickness of 100 .ANG. or larger, vertical
magnetization was not induced in the recording temperature range
due to the excessively large saturation magnetization, so that the
recording was not made due to the absence of the exchange
force.
DyCo.sub.5 (magnetic transition point 50.degree.-80.degree. C.) and
Sm.sub.0.7 Er.sub.0.3 FeO.sub.3 (magnetic transition point ca.
110.degree. C.) employed in the samples 11-3 and 11-4 provided
satisfactory recording in a thickness range of 100-400 .ANG.. This
result is based on a change of the easy axis of magnetization from
the longitudinal direction to the vertical direction in the
recording temperature range (50.degree.-150.degree. C.), despite of
the fact that the effective bias field on the second magnetic layer
is almost zero at room temperature.
Si employed in the samples 11-5 and 11-6 is not magnetic. With a
thickness of 40-60 .ANG., the Si layer hinders the exchange
coupling between the first and second magnetic layers, so that the
measured effective bias field on the second magnetic layer was as
small as 250-150 Oe. This effective bias field decreased with the
rise of temperature, so that the first-type recording, in which the
magnetization of the first magnetic layer is arranged in a
direction stabler with respect to the magnetization of the second
magnetic layer against the bias field was not possible at the
recording temperature range.
FIG. 15 shows the effective bias fields on the first or second
magnetic layer in ordinate, as a function of temperature in
abscissa, measured on the sample of the Example 10.
The first magnetic layer receives no effective bias field up to
80.degree. C., but receives, from 90.degree. C., a bias field for
orienting the magnetization of said first magnetic layer in a
stable direction with respect to the second magnetic layer. Said
bias field monotonously decreased above 90.degree. C. to reach zero
at the Curie point of the first magnetic layer. The second magnetic
layer received no bias field over the entire temperature range
measured. These results coincide with the satisfactory recording
characteristic of the magnetooptical disk of the Example 10.
Then a sample was prepared by sputtering an adjusting layer of
Tb.sub.35 Fe.sub.60 Co.sub.5, employed in the Example 10, with a
thickness of 1000 .ANG. on a slide glass, and by forming a Si.sub.3
N.sub.4 protective layer of 1000 .ANG. in thickness thereon. FIG.
16 shows the external magnetic field required to orient the
magnetization of said Tb.sub.35 Fe.sub.60 Co.sub.5 layer into the
vertical direction, in ordinate, as a function of temperature in
abscissa, measured on the above-mentioned sample. The magnitude of
said required external field decreases with the rise of
temperature, and becomes about 500 Oe in the temperature range of
80.degree.-90.degree. C. where the first magnetic layer starts to
receive a large bias field. In the sample of the Example 10, it is
assumed that the magnetization of the adjusting layer is oriented,
with the rise of temperature, in the vertical direction at the
interface between the first magnetic layer and the adjusting layer
owing to the exchange force at said interface, and the
magnetization of the adjusting layer in the vicinity of the
interface with the second magnetic layer is also oriented in the
vertical direction in a temperature range of 80.degree.-90.degree.
C., so that a large effective bias field emerges in this state
between the first and second magnetic layers through the adjusting
layer.
The above-mentioned adjusting layer, composed of a rare
earth-transition metal alloy, can be optimized in composition, in
consideration of the following factors:
(i) The rare earth-transition metal alloy shows a magnetic
anisotropy in the vertical direction, when the rare earth element
represents a range 12-28 atomic % in the rare earth and transition
metal elements. Outside said range, the easy axis of magnetization
is in the longitudinal direction or in the direction of surface,
possibly because of following two reasons. Firstly, the condition
H.sub.k .gtoreq.4.pi.M.sub.s for realizing a vertical magnetization
cannot be satisfied, because of the large saturation magnetization
M.sub.s, wherein H.sub.k being the vertical anisotropic magnetic
field. Secondly, the vertical magnetic anisotropy in a rare
earth-transition metal alloy film is caused by the coupling of the
rare earth element and the transition metal element. The magnetic
moment of the rare earth-transition metal element pair has a high
probability of orientation in the vertical direction only in the
above-mentioned percentage.
(ii) The rare earth-transition metal alloy employed in the
adjusting layer, if rich in the rare earth element compared with
the compensation composition, is increased in the rare earth
content from a composition showing vertical magnetic anisotropy
thereby increasing the saturation magnetization and facilitating
the magnetization in the longitudinal direction. If the alloy is
rich in the transition metal element compared with the compensation
composition, the transition metal is increased further from the
composition showing vertical magnetic anisotropy, thereby
increasing the saturation magnetization and facilitating the
magnetization in the longitudinal direction. If the curie point of
the material is selected in the vicinity of the recording
temperature, the saturation magnetization decreases in the vicinity
of the recording temperature, thereby satisfying the condition
H.sub.k .gtoreq.4.pi.M.sub.s for vertical magnetization. In this
manner it is possible to orient the easy axis of magnetization in
the surface direction at room temperature and in the vertical
direction at the recording temperature.
The change of the easy axis of magnetization of the adjusting layer
from the surface or longitudinal direction to the vertical
direction was experimentally confirmed in the following manner.
Three samples were prepared by sputtering, on glass substrates,
magnetic layers of Fe, Tb.sub.5 Gd.sub.5 Fe.sub.90 or Tb.sub.16
Gd.sub.16 Fe.sub.68 of a thickness of 500 .ANG. as an adjusting
layer, under an argon pressure of 5.times.10.sup.-3 Torr. On each
sample a second magnetic layer of Tb.sub.24 Fe.sub.74 Co.sub.6 of a
thickness of 500 .ANG. was formed without breaking the vacuum, and
a Si.sub.3 N.sub.4 protective layer of 700 .ANG. in thickness was
formed thereon.
Each sample was subjected to the measurement of the external
magnetic field required to orient the magnetization of the
adjusting layer into the vertical direction, as a function of
temperature.
Fe, Tb.sub.5 Gd.sub.5 Fe.sub.90 or Tb.sub.16 Gd.sub.16 Fe.sub.68
employed as the adjusting layer did not have a vertical easy axis
of magnetization at room temperature.
FIG. 17 shows the external magnetic field required for vertical
orientation in ordinate, as a function of the temperature in
abscissa, obtained in said measurement.
The adjusting layer composed of Fe did not show orientation of the
magnetization into the vertical direction, since the decrease of
saturation magnetization is still small at 160.degree. C. The
Tb.sub.5 Gd.sub.5 Fe.sub.90 and Tb.sub.16 Gd.sub.16 Fe.sub.68, both
being rare earth-transition metal alloy and having the easy
direction of magnetization also in the surface direction, show a
significant decrease in magnetization because the Curie point is in
a range of 100.degree.-200.degree. C. Thus the magnetization can be
oriented in the vertical direction with a limited external magnetic
field when heated to about 100.degree. C. Particularly Tb.sub.16
Gd.sub.16 Fe.sub.68, which is richer in the rare earth element
compared to the compensation composition, shows easier orientation
of the magnetization in the vertical direction with a smaller
external field at a higher temperature, in comparison with Tb.sub.5
Gd.sub.5 Fe.sub.90 which is richer in the transition metal
element.
Besides the required external field becomes smaller, in the
temperature range of 70.degree.-80.degree. C., than the coercive
force of the second magnetic layer. A measurement for identifying
whether the external magnetic field required for inverting the
magnetization of the adjusting layer is dependent on the direction
of magnetization of the second magnetic layer, namely whether an
exchange force exists between the adjusting layer and the second
magnetic layer, clarified that no exchange force was present at
room temperature but a bias field caused by an exchange force of
ca. 200 Oe was present at 90.degree. and 110.degree. C.
Tb.sub.16 Gd.sub.16 Fe.sub.68, richer in the rare earth element
than the compensation composition, shows an enhanced orientation of
the easy direction of magnetization in the vertical direction at
higher temperatures because of the following two reasons.
Firstly, it is empirically known that, in an exchange-coupled
combination of the adjusting layer and the second magnetic layer, a
stronger exchange force is obtained in a combination in which both
layers are rich in the rare earth element or in the transition
metal than in a combination in which one layer is rich in the rare
earth element while the other is rich in the transition metal. Thus
the adjusting layer is earily oriented in the vertical direction,
since the second magnetic layer is a magnetic film vertically
oriented to the film surface.
Secondly, the rare earth elements have lower Curie points in
isolated state. Thus, in the rare earth-transition metal alloys,
the rare earth element contributes more significantly to the
decrease of magnetization at higher temperatures, if the
composition is rich in the rare earth element. For this reason the
compensation temperature is present above room temperature.
A composition not showing vertical easy direction of magnetization
because of the excessively large magnetization of the rare earth
element shows a decrease of magnetization of said rare earth
element at higher temperatures, so that the magnetization is
represented by the magnetizations of the rare earth element and the
transition metal which originally have vertical magnetic
anisotropy.
EXAMPLE 12
A pregrooved and preformatted polycarbonate disk substrate was
placed in a sputtering apparatus with quaternary targets, and was
rotated at a distance of 10 cm from the targets
A Si.sub.3 N.sub.4 protective layer of 700 .ANG. in thickness was
formed by sputtering from a first target, in argon gas, with a
sputtering speed of 100 .ANG./min , and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbDyFeCo alloy was sputtered from a
second target, in argon gas, with a sputtering speed of 100
.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to
obtain a first magnetic layer of Tb.sub.15 Dy.sub.5 Fe.sub.76
Co.sub.4 with a thickness of 300 .ANG., T.sub.L =ca. 150.degree. C.
and H.sub.H =ca. 10 KOe. Fe and Co atoms were prevailing in the
sub-lattice magnetization of the first magnetic layer Then a TbGd
Fe alloy was sputtered from a third target, in argon gas, with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr to form an adjusting layer of Tb.sub.16
Gd.sub.16 Fe.sub.68 with a thickness of 200 .ANG., and Curie point
of ca 160.degree. C. Said adjusting layer did not show a vertical
easy direction of magnetization at room temperature, and the
external magnetic field required for orienting the magnetization
into the vertical direction was ca. 2 KOe at room temperature. Then
a TbGdFeCo alloy was sputtered from a fourth target, in argon gas,
with a sputtering speed of 100 .ANG./min. and a sputtering pressure
of 5.times.10.sup.-3 Torr to obtain a second magnetic layer of
Tb.sub.20 Gd.sub.5 Fe.sub.67 Co.sub.8 with a thickness of 300
.ANG., T.sub.H =ca. 190.degree. C. and H.sub.L =ca. 1.8 KOe. Tb and
Gd were prevailing in the sub-lattice magnetization of said second
magnetic layer.
Subsequently a Si.sub.3 N.sub.4 protective layer of 800 .ANG. in
thickness was formed by sputtering from the first target, in argon
gas, with a sputtering speed of 100 .ANG./min. and a sputtering
pressure of 5.times.10.sup.-3 Torr.
After these layer formations, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk.
Then there was made a measurement, with VSM (vibrating
magnetization measurer), of the external magnetic field inducing
the inversion of magnetization of the first and second magnetic
layers The effective bias field cause by the exchange force on the
second magnetic field was identified as almost zero.
Said magnetooptical disk was mounted on a record/reproducing
apparatus and was made to pass through, with a linear speed of ca.
8 m/sec., a unit generating a magnetic field of 2.5 KOe. The
recording operation was then conducted with a laser beam of a
wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in
two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency
of 2 MHz. The bias field at recording was 150 Oe. Binary signals
could reproduced by irradiation with a laser beam of 1.0 mW.
The above-explained experiment was repeated with a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
so that the possibility of overwriting was thus confirmed.
EXAMPLE 13
Samples of the magnetooptical disk were prepared with the same
process and materials as in the Example 12, except that the
composition of the adjusting layer was modified.
Then there were measured the effective bias magnetic field caused
by the exchange force on the second magnetic layer, and the
recording characteristic, according to the same methods as in the
Example 12. The results are summarized in Tab. 5.
TABLE 5
__________________________________________________________________________
Effective Easy direc- Coercive bias field Recording tion of mag-
force of on 2nd mag. character- Example Adjusting layer Note
netization adj. layer layer istic
__________________________________________________________________________
13-1 Tb.sub.2.5 Gd.sub.2.5 Fe.sub.95 Fe rich Surface -- 0 Not good
(1st type recording not possible) 13-2 Tb.sub.5 Gd.sub.5 Fe.sub.90
" " -- 0 Not good (1st type recording sensi- tivity low) 13-3
Tb.sub.7 Gd.sub.7 Fe.sub.86 " Vertical 0.3 KOe 0.5 KOe Not good
(1st type recording not possible) 13-4 Tb.sub.9 Gd.sub.9 Fe.sub.82
" " 1.5 2.5 Not good (1st type recording not possible) 13-5
Tb.sub.10 Gd.sub.10 Fe.sub.80 " " 7 2.5 Not good (1st type
recording not possible) 13-6 Tb.sub.12 Gd.sub.12 Fe.sub.76 Tb, Gd
rich " 5 2.5 Not good (1st type recording not possible) 13-7
Tb.sub.13 Gd.sub.13 Fe.sub.74 " " 1 2.0 Not good (1st type
recording not possible) 13-8 Tb.sub.15 Gd.sub.15 Fe.sub.70 "
Surface -- 0 Good 13-9 Tb.sub.20 Gd.sub.20 Fe.sub.60 " " -- 0 Good
13-10 Tb.sub.25 Gd.sub.25 Fe.sub.50 " " -- 0 Good 13-11 Tb.sub.40
Gd.sub.40 Fe.sub.20 " " -- 0.1 Not good (1st type recording sensi-
tivity low)
__________________________________________________________________________
In Tab. 5, the column "Note" indicates whether the composition of
the adjusting layer is rich in Fe or in Tb and Gd.
Satisfactory recording characteristic was found in the samples 13-8
to 13-10, in which the rare earth elements (Tb, Gd) represented
20-50 atomic % in the adjusting layer.
All the samples were easy to magnetize in the surface direction, as
these compositions were rich in the rare earth elements of Tb and
Gd, compared to the compensation composition.
In the samples 13-1 to 13-3, the effective bias field on the second
magnetic layer was almost zero because the adjusting layer was rich
in the transition metal (Fe), but the recording of first type could
not be achieved in stable manner, because the bias field caused by
the exchange force between the first and second magnetic layers and
acting to orient the magnetization of these layers in stable
directions was small.
The samples 13-4-13-7, having a vertically magnetized adjusting
layer, shows an effective bias field on the second magnetic layer
in the order of 2.0 to 2.5 KOe, larger than the coercive force
thereof. The recording of first type could be made in these
samples, since the magnetization of the second magnetic layer is
always oriented in a stable direction with respect to the
magnetization of the first magnetic layer.
The sample 13-8 had the highest proportion of the rare earth
element and a Curie point lower than 100.degree. C. The recording
of first type could be made satisfactorily because there was no
sufficient bias field caused by the exchange force between the
first and second magnetic layers at the recording temperature.
EXAMPLE 14
In the samples of the Example 13, the compositions of the first
magnetic layer, adjusting layer and second magnetic layer were
respectively rich in the transition metal, rich in rare earth
element and rich in rare earth element.
In the present example, samples were prepared with same
thicknesses, materials and structures as those in the Example 13,
except that the first and second magnetic layers and the adjusting
layer were selected from a transition metal-rich composition (TM)
and a rare earth-rich composition (RE) of a same coercive force and
a same Curie temperature. The samples were evaluated in the same
manner as in the Example 13, as summarized in Tab. 6.
TABLE 6
__________________________________________________________________________
Structure Effect. bias 1st mag. Adjust. 2nd mag. field on 2nd
Recording Sample layer layer layer mag. layer characteristic
__________________________________________________________________________
14-1 TM TM TM 1 KOe Not good (1st type recording not possible) 14-2
TM RE TM 0 Not good (1st type recording sensi- tivity low) 14-3 RE
TM TM 0 Not good (1st type recording sensi- tivity low) 14-4 RE RE
TM 0 Not good (1st type recording sensi- tivity low) 14-5 RE TM RE
0 Not good (1st type recording unstable) 14-6 RE RE RE 1 Not good
(1st type recording not possible) 14-7 TM TM RE 0 Not good (1st
type recording sensi- tivity low)
__________________________________________________________________________
TABLE 7 ______________________________________ Coercive Curie Layer
Type Composition force point ______________________________________
1st mag. layer TM Tb.sub.15 Dy.sub.5 Fe.sub.76 Co.sub.4 10 KOe
150.degree. C. RE Tb.sub.16.5 Dy.sub.5.5 Fe.sub.74 Co.sub.4 10
150.degree. C. Field for vertical orientation Adjust. layer TM
Tb.sub.6 Gd.sub.6 Fe.sub.88 2 KOe 150.degree. C. RE Tb.sub.16
Gd.sub.16 Fe.sub.68 2 150.degree. C. 2nd mag. layer TM Tb.sub.12
Gd.sub.3 Fe.sub.80 Co.sub.5 1.8 190.degree. C. RE Tb.sub.20
Gd.sub.5 Fe.sub.67 Co.sub.8 1.8 190.degree. C.
______________________________________
In Tab. 6 there are shown, corresponding to the coercive force of
the adjusting layer, the values of external magnetic field required
for orienting the magnetization into the vertical direction. As
will be apparent from Tab. 7, the samples 14-1 to 14-4, with
transition metal-rich second magnetic layer, showed a larger
decrease in the coercive force at an elevated temperature than in
the samples of the Example 13 with rare earth-rich composition, so
that the coercive force of the second magnetic layer became
insufficient for forming stable bits at the temperature of
recording of first type. For this reason there was encountered a
low sensitivity for the recording of first type or unstable
recording.
The samples 14-3 to 14-6, with rare earth-rich first magnetic
layers, showed a smaller decrease of the coercive force at the
elevated temperature, in comparison with the transition metal-rich
compositions in the Example 13. Consequently the condition H.sub.H
-H.sub.Heff <0 for recording in the first magnetic layer at the
estimated recording temperature of the first type. Thus the
recording of first type could not be conducted, or the sensitivity
therefor was low.
Also the samples 14-1, 14-2, 14-5 and 14-6, in which the first and
second magnetic layers were both rich in the rare earch element or
in the transition metal, showed a strong exchange force even at a
relatively low temperature when the medium was heated, and the
recording of first type was only conducted unstable in all these
samples, because the magnetization of the second magnetic layer was
oriented in the stable direction with respect to that of the first
magnetic layer.
The samples 14-1, 14-3, 14-5 and 14-7, with transition metal-rich
adjusting layers, showed a smaller bias magnetic field caused by
the exchange force acting on the first and second magnetic layers
through the adjusting layer, in comparison with the samples of the
Example 13 with rare earth-rich adjusting layers. Consequently the
sensitivity for the recording of first type was lowered.
From the foregoing results it can be concluded that the optimum
combination of the compositions of the first and second magnetic
layers and the adjusting layer corresponds to the case of the
Example 13, in which the first magnetic layer, adjusting layer and
second magnetic layer are respectively rich in transition metal,
rare earth element and rare earth element.
In the foregoing examples, the recording film has triple-layered
structure consisting of the first and second magnetic layers and
the adjusting layer, but it is also possible to form, between the
first magnetic layer and the substrate, a fourth magnetic layer of
a strong magnetooptical effect in strong exchange-coupling with the
first magnetic layer. In such case the sum of the thicknesses of
said first and fourth magnetic layers is preferably in excess of
200 .ANG. for increasing the reproduction output.
In the following there will be given more detailed explanation on
the structure of the magnetooptical recording medium of the present
invention and variations thereof.
Thickness of Magnetic Layer
In the medium shown in FIG. 3, the thickness L1 of the first
magnetic layer 2 and the thickness L2 of the second magnetic layer
3 are preferably determined in the following manner.
In the conventional exchange-coupled double-layered film, the
magnetization of a record bit in either of two states is inverted
in two magnetic layers according to the state to be recorded. On
the other hand, in the present invention, the magnetization of the
first magnetic layer 2, principally related to the reproduction, is
inverted according to the state to be recorded, while the
magnetization of the second magnetic layer 3, principally related
to the recording, coincides with the direction of the field from
the magnetic field generating unit 8 shown in FIG. 6 and remains
unchanged. Since the sensitivity of the magnetooptical recording
medium is lowered as the thickness of the magnetic layer not
contributing to the magnetooptical effect increases, it is
necessary to minimize the total thickness L1+L2 of said magnetic
film and to optimize the combination of L1 and L2 so as to increase
the magnetooptical effect at the reproduction. As will be explained
in the following examples, the layer thicknesses should satisfy
following conditions:
in order to obtain a satisfactory sensitivity and a large
magnetooptical effect.
EXAMPLE 15
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with ternary targets, and was rotated at
a distance of 10 cm from the targets.
A ZnS protective layer of 800 .ANG. in thickness was formed by
sputtering from a first target, in argon gas, with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered, in argon
gas, from a second target with a sputtering speed of 100 .ANG./min.
and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a
first magnetic layer of Tb.sub.19.5 Fe.sub.80.5 with a thickness of
300 .ANG., T.sub.L =ca. 140.degree. C. and H.sub.H =ca. 8 KOe.
Then a TbFeCo alloy was sputtered, in argon gas, with a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic
layer of Tb.sub.24.5 Fe.sub.68 Co.sub.7.5 with a thickness of 400
.ANG., T.sub.H =ca. 190.degree. C. and H.sub.L =ca. 0.8 KOe.
Subsequently a ZnS protective layer of 3000 .ANG. in thickness was
formed by sputtering, in argon gas, from the first target with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After these layer formations, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. (600 .ANG.<L1+L2=700 .ANG.<1000
.ANG., 200 .ANG.<L1=300 .ANG.)
Said magnetooptical disk was set on a record/reproducing apparatus,
and was made to pass through, with a linear speed of ca. 8 m/sec.,
a unit for generating a magnetic field of 2.5 KOe. The recording
operation was then conducted with a laser beam of a wavelength of
830 nm, concentrated to ca. 1 .mu.m and modulated in two levels of
4 and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The
bias field was 100 Oe.
Binary signals could be reproduced by irradiation with a laser beam
of 1.5 mW.
The above-explained experiment was repeated on a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
and the possibility of overwriting was thus confirmed.
EXAMPLE 16
Samples of magnetooptical disk were prepared with the same process
and materials as those in the Example 15, except that the
thicknesses of the first and second magnetic layers were
modified.
For evaluating the reproduction signal from each sample, the
reflectance at 830 nm and the Kerr rotation angle were measured.
Also there was calculated the product of aquare root of reflectance
and Kerr rotation angle, which is usually accepted as an index for
the reproducing performance in case the photosensor is composed of
a photodiode. These results are summarized in Tab. 8.
TABLE 8 ##STR1## ##STR2## ##STR3##
In the record bit of the present invention, the magnetization of
the second magnetic layer is not changed by the state of record and
does not contribute to the magnetooptical effect. In case the first
magnetic layer 2 is thin, the intensity of reflected light is
reduced and the aforementioned performance index is also reduced if
the second magnetic layer 3 is thin or absent.
The intensity of the reflected light becomes almost constant if the
total thickness L1+L2 of the first and second magnetic layers 2, 3
exceeds 400 .ANG..
It is also found out that said performance index and Kerr rotation
angle become saturated at substantially constant values when the
total thickness L1+L2 of the first and second magnetic layers is
equal to or larger than 600 .ANG., but, if the thickness L1 of the
first magnetic layer 2 is less than 200 .ANG., the Kerr rotation
angle is saturated at a smaller value than the saturation value
when said thickness L1 is larger than 200 .ANG. even when the
thickness L2 of the second magnetic layer 3 is increased. From
these results it is concluded that the conditions 600
.ANG.<L1+L2 and 200 .ANG.<L1 are preferable.
These samples were subjected to a test of recording and
reproduction in the same manner as in the Example 15, and
satisfactory reproduction signal as in the Example 15 was obtained
from the samples in which L1+L2 was equal to or larger than 600
.ANG..
On the other hand, when L1+L2 was equal to 1000 .ANG., the laser
powers required for the recordings of first and second types were
respectively 2.5 times of those required when L1+L2 was 600 .ANG.,
and said powers rapidly increased when L1+L2 exceeded 1000 .ANG..
Thus a condition L1+L2 <1000 .ANG. was concluded preferable.
As explained in the foregoing, though the second magnetic layer 3
does not change its magnetization during the reproduction of the
recorded signals of first and second types, thus not contributing
to the magnetooptical effect, the presence of said second magnetic
layer provides a Kerr rotation angle and a reproduction performance
index equivalent to the case where the first magnetic layer 2 has a
thickness L1+L2.
For the purpose of comparison, the reflectance, Kerr rotation angle
and performance index were determined by applying a field stronger
than the coercive force of the first magnetic layer in each sample,
thereby inverting the magnetization of the second magnetic layer 3
as well as that of the first magnetic layer both in the recordings
of first and second types, just as in the conventional
exchange-coupled double-layered magnetic film. The results are
summarized in Tab. 9.
TABLE 9 ##STR4## ##STR5## ##STR6##
In this case, large rotation angle and index can be obtained if the
thickness of the second magnetic layer is in a range of 300-400
.ANG., even if the first magnetic layer 2 is thinner than 200
.ANG., because the second magnetic layer 3 shows a magnetooptical
effect induced by the inversion of magnetization, and because the
second magnetic layer 3 has a higher Curie point and shows a larger
Kerr rotation angle than the first magnetic layer 2 at a same
thickness. However the contribution of the second magnetic layer 3
in the magnetooptical effect is negligible when the thickness of
the first magnetic layer 2 exceeds 200-250 .ANG..
These results coincide with those of Tab. 8 that the presence of
the second magnetic layer 3 provides a Kerr rotation angle and a
performance index equivalent to those when the thickness of the
first magnetic layer 2 is equal to L1+L2.
Also when the ZnS protective layers in the samples of the Example
15 are replaced by those of Si.sub.3 N.sub.4, SiC, SiO or Al.sub.2
O.sub.3, the relationship of the thicknesses of the first and
second magnetic layers providing saturated rotation angle and
performance index remains unchanged. SiC, having - refractive index
larger than that of Si.sub.3 N.sub.4 or ZnS, increased the index by
about 10%, but the recording sensitivity was lowered for a same
thickness L1+L2. On the other hand, SiO or Al.sub.2 O.sub.3, having
refractive indexes lower than those of Si.sub.3 N.sub.4 or ZnS,
decreased the performance index by about 10%.
Also the relationship of the thicknesses of the first and second
magnetic layers providing saturated rotation angle and performance
index was not affected by a change of the material constituting the
first magnetic layer 2, from TbFe to GdTbFe, TdFeCo, GdTbFeCo or
DyTbFeCo.
Compensation Temperature of Magnetic Layers
In the magnetooptical recording medium as shown in FIG. 3, the
compensation temperature T.sub.Hcomp of the second magnetic layer 3
is preferably positioned between room temperature and Curie
temperature T.sub.H of said second magnetic layer. Also such medium
allows a recording process utilizing said compensation temperature.
This process will be explained in the following, separately in a
case of T.sub.L <T.sub.Hcomp and a case of T.sub.Hcomp
<T.sub.L.
(i) Case T.sub.L <T.sub.Hcomp
Referring to FIG. 5, in one of the states 4b prior to the first
preliminary recording, in which the magnetizations of the first and
second magnetic layers are in a same direction, said magnetic
layers are stabilized by the exchange force, so that the
magnetization of the first magnetic layer can only be inverted by a
magnetic field H.sub.H '+H.sub.Heff ', which are respectively the
coercive force of the first magnetic layer and the exchange force
acting on said layer at a temperature t and which are both
functions of the temperature t, while the magnetization of the
second magnetic layer can only be inverted by a magnetic field
H.sub.L '+H.sub.Leff ', which are respectively the coercive force
of the second magnetic layer and the exchange force acting on said
layer at a temperature t and which are both functions of the
temperature t. For this reason the preliminary recording bit 4c can
be stably formed, even in the presence of a certain bias field in
either direction. However, in the other of the states 4b shown in
FIG. 5, in which the magnetizations of the first and second
magnetic layers are mutually oppositely directed, each magnetic
layer receives an exchange force to invert its magnetization. Thus,
there will result a situation (a) or (b) explained in the
following.
(a) If a situation H.sub.H '-H.sub.Heff '<0 occurs when the
magnetic layers are heated close to T.sub.L at the preliminary
recording of first type, the magnetization of the first magnetic
layer is inverted and arranged in a stable direction with respect
to the magnetization of the second magnetic layer, thus completing
the preliminary recording of first type.
(b) On the other hand, if a condition H.sub.L '-H.sub.Leff '<0
is reached before the inversion of magnetization of the first
magnetic layer, the magnetization of the second magnetic layer is
inverted and arranged in a stable direction with respect to the
magnetization of the first magnetic layer. Therefore, the
preliminary recording of first type, requiring the inversion of
magnetization of the first magnetic layer, becomes impossible.
Therefore the value of H.sub.L ' should be increased as far as
possible, and, for this purpose, H.sub.L should be maximized, but
can only be increased to ca. 2 KOe because the second magnetic
layer needs to be uniformly magnetized by the magnetic field
generating unit 8 shown in FIG. 6. Consequently, in order to
maximize the coercive force of the second magnetic layer in the
vicinity of the temperature T.sub.L, it is important to select the
compensation temperature T.sub.Hcomp of the second magnetic layer
close to T.sub.L.
In this manner it is rendered possible to prevent the decrease in
the coercive force H.sub.L ' of the second magnetic layer in the
course of heating toward the compensation temperature T.sub.Hcomp,
because the coercive force of the second magnetic layer becomes
infinite at the compensation temperature T.sub.Hcomp even if the
saturation magnetization M.sub.s of the second magnetic layer is
decreased.
As will be apparent from the following examples, it is possible to
achieve the preliminary recording of the first type in stable
manner, by selecting a second magnetic layer satisfying conditions:
##EQU6##
Among the aforementioned substances, the above-mentioned conditions
can be satisfied by a TbGdCo alloy richer in Tb than the
compensation composition. Also there may be employed substances
obtaining by adding suitable impurities such as another rare earth
element or another transition metal to the above-mentioned ternary
compound.
EXAMPLE 17
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with ternary targets, and was rotated at
a distance of 10 cm from the targets.
After the sputtering apparatus was evacuated to 1.times.10.sup.-6
Torr or lower, a ZnS protective layer of 1000 .ANG. in thickness
was formed by sputtering, in argon gas, from a first target with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered, in argon
gas, from a second target with a sputtering speed of 100 .ANG./min.
and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a
first magnetic layer of Tb.sub.18 Fe.sub.82, with a thickness of
300 .ANG., T.sub.L =ca. 130.degree. C. and H.sub.H =ca. 10 KOe.
Then a TbFeCo alloy was sputtered, in argon gas, with a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic
layer of Tb.sub.27 Fe.sub.64 Co.sub.9 with a thickness of 500
.ANG., T.sub.H =ca. 190.degree. C., T.sub.Hcomp =210.degree. C. and
H.sub.L =ca. 1 KOe.
Subsequently a ZnS protective layer of 3000 .ANG. in thickness was
formed by sputtering, in argon gas, from the first target with a
sputtering speed of .ANG. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After these layer formations, the above-mentioned substrate was
adhered with a polycarbonate plate with hot-melt adhesive material
to obtain a magnetooptical disk.
The second magnetic layer showed a coercive force of ca. 700 Oe
around the Curie point (130.degree. C.) of the first magnetic
layer, when measured by the method explained in the Example 18.
This magnetooptical disk was mounted on a record/reproducing
apparatus, and was made to pass through, with a linear speed of ca.
8 m/sec., a unit generating a magnetic field of 2.5 KOe. The
recording was conducted with a laser beam of a wavelength of 830
nm, concentrated to ca. 1 .mu.m and modulated in two levels of 4
and 8 mW, with a duty ratio of 50% and a frequency of 2 MHz. The
bias field was 100 Oe in a direction to invert the magnetization of
the second magnetic layer. Binary signals could be reproduced by
irradiation with a laser beam of 1.5 mW.
The above-explained experiment was repeated with a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
and this result confirms that the overwriting is possible.
EXAMPLE 18
According to a process similar to that in the Example 17, there
were prepared a magnetooptical disk sample 18-1 having a ZnS layer
of 1000 .ANG., a first magnetic layer of Tb.sub.18 Fe.sub.82 of 300
.ANG., a second magnetic layer of Tb.sub.27 Fe.sub.64 Co.sub.9 of
500 .ANG., and a ZnS layer 1000 .ANG. formed in this order on a
glass substrate, and a sample 18-2 of the same structure except
that the second magnetic layer was replaced by Tb.sub.15 Fe.sub.81
Co.sub.4.
B-H loops were determined by measuring the magnetic inducing the
inversion of magnetization, by
Kerr effect, in the samples 18-1 and 18-2, at various temperatures.
In this manner obtained were the temperature characteristic of the
coercive force (FIG. 18) and the temperature dependence of the
difference of coercive force and exchange force (FIG. 19), for the
Tb.sub.18 Fe.sub.82 layer (hereinafter called layer A), Tb.sub.27
Fe.sub.64 Co.sub.9 layer (layer B) and Tb.sub.15 Fe.sub.81 Co.sub.4
(layer C).
As shown in FIG. 18, the layer A showed, at room temperature, a
coercive force H.sub.H of ca. 10 KOe and a Curie point T.sub.1 of
ca. 130.degree. C.
Also at room temperature, the layers B and C showed a coercive
force H.sub.L of ca. 1 KOe and a curie point T.sub.H of ca.
190.degree. C.
The compensation temperature was ca. 210.degree. C. and ca.
-130.degree. C. respectively for the layers B and C. The forces of
the layers B and C around T.sub.L were respectively ca. 70% and ca.
30% of those at room temperature.
In FIG. 19 showing the temperature dependence of the difference of
coercive force and exchange force, broken lines indicate H.sub.L
'-H.sub.HEFF ' for the layer B or C, while solid lines indicate
H.sub.H '-H.sub.Heff ' for the first magnetic layer A.
The lines marked as A/B are related to the sample 18-1, while those
marked as A/C are related to the sample 18-2.
In practice, only the smaller one of H.sub.H '-H.sub.Heff ' and
H.sub.L '-H.sub.Leff ' is measurable. The larger one cannot be
measured because, when the external magnetic field is increased,
the magnetization of a magnetic layer with a smaller coercivity is
inverted at a point lower than the inverting magnetic field.
The meaning of Gi.19 will be explained in the following.
When the inverting magnetic field is measured with increasing
temperature, H.sub.L '-H.sub.Leff ' of the second magnetic layer
initially has a positive value of 200-3000 Oe. However, at a
temperature lower than T.sub.L by 20.degree.-50.degree. C., H.sub.H
'-H.sub.Heff ' of the first magnetic layer becomes negative, so
that the second magnetic layer is spontaneously oriented in the
stable direction with respect to the magnetization of the first
magnetic layer. At a temperature higher than T.sub.L, the first
magnetic layer is no longer magnetized to annulate the exchange
force, so that only the coercive force of the second magnetic layer
is measured.
While the sample 18-1 starts the preliminary recording of the first
type from a temperature lower than T.sub.L by about 50.degree. C.
and shows the inversion of magnetization of the first magnetic
layer due to a strong exchange force, the sample 18-2 starts said
preliminary recording at a temperature lower than T.sub.L by about
20.degree. C. due to a weaker exchange force.
This difference results from a difference in the
temperature-dependent coercive force of the second magnetic layer,
or in the compensation temperature. The value of (coercive
force-exchange force) of the first magnetic layer changes from a
large positive value to a negative value as the sample temperature
is increased. Therefore, if the value of (coercive force-exchange
force) of the second magnetic value is a relatively large positive
value at the temperature range of preliminary recording of the
first type, the first magnetic layer receives a strong exchange
force (value of (coercive force-exchange force) being a large
negative value) from a relatively low temperature at the
preliminary recording of the first type, thereby orienting the
magnetization into a stable direction with respect to the
magnetization of the second magnetic layer.
In conclusion, it is concluded that the temperature dependence of
the coercive force of the second magnetic layer and the magnitude
relationship between the compensation temperature of the second
magnetic layer and the Curie temperature of the first magnetic
layer are the factors determining the stability and sensitivity of
the preliminary recording of first type.
EXAMPLE 19
Samples 19-1 to 19-6 of magnetooptical disk were prepared with the
same process, structure and thicknesses as in the Example 17,
except that the substance of the second magnetic layer was changed,
and subjected to the evaluation of recording and reproduction in
the same conditions as in the Example 17. The results are shown in
Tab. 10.
TABLE 10
__________________________________________________________________________
Compensation Ratio of Threshold Evaluation Evaluation Composition
of T.sub.H of 2nd temp. of 2nd coercive force value for of first of
second 2nd mag. layer mag. layer mag. layer at T.sub.L to room
first type type record type record Example (atom. %) (.degree.C.)
T.sub.Hcomp temp. recording at 4 mW at 8 mW
__________________________________________________________________________
Ex. 17 Tb.sub.27 Fe.sub.64 Co.sub.9 190 210 0.7 3.5 mW + + 19-1
Tb.sub.15 Fe.sub.81 Co.sub.4 190 below room 0.3 4.5 mW - + temp.
19-2 Tb.sub.15 Fe.sub.71.5 Co.sub.13.5 240 below room 0.5 4 mW .+-.
.+-. temp. 19-3 Tb.sub.15 Fe.sub.66 Co.sub.19 280 below room 0.7
3.5 mW + - temp. 19-4 Tb.sub.27 Fe.sub.67 Co.sub.6 170 240 0.7 3.5
mW + + 19-5 Tb.sub.27 Fe.sub.61 Co.sub.12 215 210 0.8 3.5 mW + .+-.
19-6 Gd.sub.13 Tb.sub.14 Fe.sub.64 Co.sub.9 197 220 0.8 3.5 mW + +
__________________________________________________________________________
However the coercive force H.sub.L of the second magnetic layer is
about 1 KOe in all the samples. In Tab. 10, "Ratio of coercive
force at T.sub.L to room temp." indicates the coercive force at the
temperature T.sub.L of the first magnetic layer (ca. 130.degree. C.
in the present example) divided by the coercive force at room
temperature.
The threshold value for the recording of first type indicates the
laser power enabling the recording.
The recording of first or second type was evaluated as "+" if a
satisfactory reproduction signal of a C/N ratio of about 40 dB
could be obtained, as ".+-." if the recorded signal could be
confirmed, and as "-" if recording was not made.
As shown in Tab. 10, the recording of first type could be
satisfactorily made only in the samples having the ratio of
coercive force at T.sub.L to that at room temperature is equal to
or larger than 0.5 (samples 17, 19-2 to 19-6), in which the
compensation temperature of the second magnetic layer is higher
than the Curie temperature of the first magnetic layer, or the
Curie temperature T.sub.H of the second magnetic layer is higher
than that in other samples (case of samples 19-2 and 19-3).
Similarly the recording of second type could be satisfactorily made
in the samples having the Curie temperature T.sub.H of the second
magnetic layer lower than 200.degree. C. (samples 17, 19-4, 19-5
and 19-6).
Also a medium with T.sub.L <T.sub.Hcomp explained above can be
utilized in a recording process as shown in FIG. 20, in which there
are shown a first magnetic layer 2 and a second magnetic layer 3.
Various states of magnetization of said layers are represented by
44a-44g. In the recording process, at a position different from the
recording head, there is applied a downward external magnetic field
H.sub.E of a magnitude enough for magnetizing the second magnetic
layer of a coercive force H.sub.L in one direction but insufficient
for inverting the magnetization of the first magnetic layer of a
coercive force H.sub.H, and a downward bias magnetic field H.sub.B
for facilitating the recording in the second magnetic layer is
applied at the position of the recording head.
Prior to the explanation of the steps of said recording process,
there will be briefly explained the states 44a-44g and the
transitions therebetween.
44a and 44g indicate two different record states at room
temperature. Heating with a laser beam causes a transition in the
order of 44b, 44c and 44d. 44b and 44f, or 44c and 44e represent
different states at a substantially same temperature. An arrow
.rarw..fwdarw. indicates a reversible magnetizing process, while
arrows .rarw. and .fwdarw. indicate irreversible magnetizing
processes. The compensation temperature of the second magnetic
layer is positioned between 44b and 44c, or between 44c and 44f.
FIG. 20 shows a case in which the rare earth lattice magnetization
is prevailing in the first and second magnetic layers. In such
case, the state 44g in which the magnetizations of the layers are
mutually in a same direction is stable, while the state 44a in
which the magnetizations are mutually oppositely directed is
unstable, due to the mutual exchange effect of the two layers, and
an interfacial magnetic wall exists in said unstable state 44a.
However the coercive force of the second magnetic layer has to be
so adjusted as that said unstable state can be maintained even in
the absence of the magnetic field. In the room temperature state
(44a, 44g), the magnetization of the second magnetic layer with
smaller coercive force is always oriented downwards by the external
magnetic field H.sub.E.
In the following there will be explained the steps of the recording
process.
When the temperature is raised from the state 44a, the coercive
force of the first magnetic layer decreases while that of the
second magnetic layer increases as shown in FIG. 22. As the
magnetizations of two layers tend to be oriented in a same
direction by the exchange force, the magnetization of the first
magnetic layer is inverted downwards as shown by 44b. If the
temperature is lowered from this state, the magnetization remains
unchanged to reach the state 44g. After the temperature is raised
from the state 44g to reach the state 44b, the state 44g is
re-gained even if the temperature is decreased. That is, the state
44g is raised from either one of the state 44a and the state 44g by
the application of a laser power heating the magnetic layer to a
temperature at which the state 44b is reached.
If the temperature is further raised from the state 44b to a state
44c across the compensation temperature T.sub.comp of the second
magnetic layer, the magnetization thereof is reversibly inverted.
If the temperature is further elevated, the coercive force of the
second magnetic layer is reduced, so that the magnetization thereof
is inverted by the bias field H.sub.B as shown by 44d. If the
temperature is lowered from this state, the magnetization remains
unchanged, and the magnetization of the second magnetic layer is
reversibly inverted when crossing the compensation temperature
T.sub.comp. At the same time the first magnetic layer starts to
show upward magnetization due to the exchange force. The coercive
force of the second magnetic layer becomes smaller when it is
cooled to room temperature, and the magnetization thereof is
inverted by the external field H.sub.E. However, the first magnetic
layer, because of the large coercive force thereof, does not show
inversion of the magnetization by the external field H.sub.E but
retains the record state. In this manner, by the application of a
laser power corresponding to the temperature of 44 d, both states
44a and 44g are transformed to the state 44a.
Thus different states of magnetization can be obtained by the
application of different laser powers, and this is a principle of
overwriting.
FIG. 21 shows a case in which transition metal sub-lattice
magentization is prevailing in the first magnetic layer, while rare
earth sub-lattice magnetization is prevailing in the second
magnetic layer. In such case a state 45a, in which the
magentizations of both layers are mutually oppositely directed, is
a stable state, and a state 45g, in which the magnetizations of
both layers are mutually in a same direction, is an unstable state,
due to the exchange effect between the two layers, and an
interfacial magnetic wall exists in said unstable state 45g. In a
similar manner as shown in FIG. 20, the application of a laser
power corresponding to the temperature of a state 45b brings the
states 45a and 45g to the state 45a, while the application of a
laser power corresponding to the temperature of a state 45d brings
the states 45a and 45g to the state 45g. Consequently, different
states of magnetization can be again obtained by the application of
different laser powers. In this manner overwriting can be
achieved.
The magnitude of the field H.sub.B is smaller than that of the
field H.sub.E. If H.sub.B is larger than H.sub.E, the field H.sub.E
becomes unnecessary but the control of the compositions and layer
thicknesses of the medium becomes more difficult. More
specifically, if H.sub.B >H.sub.E, H.sub.B has to be
considerably large, and it is considerably difficult to realize the
state 44f in FIG. 20 or 45b in FIG. 21 by the exchange force,
against such large magnetic field. Since the second magnetic layer
has a compensation temperature as explained before, the
magnetization of the second magnetic layer in the against the
magnetic vield H.sub.B.
EXAMPLE 20
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with ternary targets, and was rotated at
a distance of 10 cm from the targets.
A Si.sub.3 N.sub.4 protective layer of 700 .ANG. in thickness was
formed by sputtering, in argon gas, from a first target with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered, in argon
gas, from a second target with a sputtering speed of 100 .ANG./min.
and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a
first magnetic layer with prevailing Tb sub-lattic magnetization,
with a thickness of 500 .ANG., T.sub.L =ca. 130.degree. C. and
H.sub.H =ca. 5 KOe.
Then a Gd-Tb-Fe-Co alloy was sputtered, in argon gas, from a third
target with a sputtering speed of 100 .ANG./min and a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic
layer with prevailing GdTb sub-lattice magnetization, with a
thickness of 800 .ANG., T.sub.H =ca. 220.degree. C., H.sub.L =ca.
1.5 KOe and T.sub.comp =ca. 140.degree. C.
Subsequently a Si.sub.3 N.sub.4 protective layer of 700 .ANG. in
thickness was formed by sputtering, in argon gas, from the first
target with a sputtering speed of 100 .ANG./min. and a sputtering
pressure of 5.times.10.sup.-3 Torr.
After these layer formations, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. Said magnetooptical disk was set on a
record/reproducing apparatus, and the recording was conducted with
a laser beam of a wavelength of 830 nm, focused to ca. 1.5 .mu.m
and modulated in two levels of 2.7 and 5.5 mW, with a duty ratio of
50% and a frequency of 2 MHz, and with a linear speed of ca. 8
m/sec and under the application of a bias magnetic field of 200 Oe
and an external magnetic field of 2 KOe. Subsequently binary
signals could be reproduced by irradiation with a laser beam of 1
mW.
After the above-explained experiment, recording was conducted on
the same track with a same power and a frequency of 3 MHz. The
previously recorded signals were not detected, and the possibility
of over-writing was thus confirmed.
EXAMPLE 21
A magnetooptical disk was prepared in the same manner as in the
Example 20, except that, for the first magnetic layer, a TbFe alloy
was sputtered from the second target with a sputtering speed of 100
.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to
obtain a magnetic layer with prevailing Fe sub-lattice
magnetization, with a thickness of 500 .ANG., T.sub.L =ca.
125.degree. C., and H.sub.H =ca. 4 KOe, and said disk was evaluated
in the same manner.
The evaluation clarified that the overwriting was possible
also.
(ii) Case T.sub.Hcomp <T.sub.L
The recording process is conducted in the following manner:
(a) To the recording medium there is applied, at a position
different from the recording head, a magnetic field B enough for
magnetizing the second mangetic layer of a coercive force H.sub.L
in a direction but insufficient for inverting the magnetization of
the first magnetic layer of a coercive force H.sub.H.
(b) Then there is conducted, according to the input signal, either
the preliminary recording of first type in which a bias magnetic
field is applied by the recording head and a laser power enough for
heating the medium close to the compensation temperature
T.sub.Hcomp is simultaneously applied, thereby orienting the
magnetzation of the first magnetic layer in a stable direction with
respect to the magnetization of the second magnetic layer without
varying said magnetization, or the preliminary recording of second
type in which a laser power enough for heating the medium close to
the higher of the Curie temperatures T.sub.L, T.sub.H of the first
and second magnetic layers is applied simultaneously with the
application of the bias magnetic field, thereby inverting the
magnetization of the second magnetic layer and simultaneously
magnetizing the first magnetic layer in a stable direction with
respect to the magnetization of said second magnetic layer.
(c) Then the medium is so moved that the preliminarily recorded bit
passes through the aforementioned magnetic field B, whereby the bit
formed by the preliminary recording of first type does not change
the direction of magnetization in the first and second magnetic
layers, while the bit formed by the preliminary recording of second
type does not change the direction of magnetization of the first
magnetic layer but inverts the magnetization of the second magnetic
layer in a direction same as that of said magnetic field B.
In this case the Curie temperatures T.sub.L, T.sub.H of the first
and second magnetic layers should satisfy a relation T.sub.L
.ltoreq.T.sub.H. On the other hand, the condition for the recording
of first type is H.sub.H '-H.sub.Heff '<0, and H.sub.H ' should
preferably decrease rapidly at elevated temperature. Consequently,
the compensation temperature T.sub.comp of the first magnetic layer
should preferably be lower than room temperature. The relation of
the compensation temperature, Curie temperature and coercive in
these magnetic layers is shown in FIG. 23.
EXAMPLE 22
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with ternary targets, and was rotated at
a distance of 10 cm from the targets.
A ZnS protective layer of 1000 .ANG. in thickness was formed by
sputtering, in argon gas, from a first target with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered, in argon
gas, from a second target with a sputtering speed of 100 .ANG./min.
and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a
first magnetic layer of Tb.sub.18 Fe.sub.82 with a thickness of 300
.ANG., T.sub.L =ca. 130.degree. C., a compensation temperature
below room temperature, and H.sub.H =ca. 10 KOe.
Then a TbFeCo alloy was sputtered, in argon gas, with a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic
layer of Tb.sub.27 Fe.sub.51 Co.sub.9 Cu.sub.13 with a thickness of
500 .ANG., T.sub.h =ca. 210.degree. C., H.sub.L =ca. 1 KOe and a
compensation temperature of 100 .degree. C.
Subsequently a ZnS protective layer of 1000 .ANG. in thickness was
formed by sputtering, in argon gas, from the first target with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After forming these layers, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk.
Thus prepared was mounted on a record/reproducing apparatus, and
was made to pass through a unit generating a magnetic field of 2.5
KOe with a linear speed of ca. 8 m/sec. The recording was then
conducted with a laser beam of a wavelength of 830 nm, concentrated
to ca. 1 .mu.m and modulated in two levels of 4 and 8 mW with a
duty ratio of 50% and a frequency of 2 MHz. The bias field was 100
Oe in a direction to invert the magnetization of the second
magnetic layer. Binary signals could then be reproduced by
irradiation with a laser beam of 1.5 mW.
The above-explained experiment was repeated with a sample disk
recorded over the entire surface. The previously recorded signal
components were not detected in the reproduction, and the
possibility of over-writing was confirmed in this manner.
EXAMPLE 23
A sample of magnetooptical disk was prepared in the same manner as
in the Example 22, except that the first magnetic layer was
composed of Tb.sub.18 Fe.sub.72 Co.sub.10, having a Curie
temperature of ca. 220.degree. C., a compensation temperature below
room temperature, and a coercive force of ca. 10 KOe.
EXAMPLE 24
A sample of magnetooptical disk was prepared in the same manner as
in the Example 22, except that the second magnetic layer was
composed of Tb.sub.15 Fe.sub.77 Co.sub.8, having a Curie
temperature of ca. 210.degree. C., a compensation temperature below
room temperature, and a coercive force of ca. 1 KOe.
The samples of the Examples 22, 23 and 24 were subjected to the
test of recording and reprocution, according to the process of the
Example 22 and with varying recording laser power. Tab. 11
summarizes the threshold values (where C/N ratio is saturated) of
the recordings of first and second types, and the corresponding C/N
ratios.
TABLE 11 ______________________________________ 1st type recording
2nd type recording Sample threshold v. C/N threshold v. C/N
______________________________________ Example 22 2 mW 46 dB 6 mW
46 dB Example 23 2.5 mW 49 dB 7 mW 49 dB Example 24 4.5 mW 35 dB 6
mW 45 dB ______________________________________
Tab. 11 shows that the samples of the Examples 22 and 23 have
higher sensitivities of the recording of first type and larger C/N
ratios compared to the sample of the Example 24. These results
reflect that the second magnetic layer has a compensation
temperature higher than room temperature and increases the coercive
force H.sub.L of the second magnetic layer in the course of
recording, whereby the second magnetic layer is stabilized with
respect to the magnetic field and the inversion of magnetization of
the first magnetic layer takes place stably from a low
temperature.
Also the comparison of the Examples 22 and 23 suggests that the
Example 23 shows an increased C/N ratio at the reproduction, though
the recording sensitivity is somewhat lost, because of the higher
Curie temperature of the first magnetic layer.
Protective Layer
FIG. 24 is a schematic cross-sectional view showing an embodiment
of the magnetooptical recording medium of the present invention,
wherein a protective layer 46 is provided on the second magnetic
layer 3. Said protective layer 46 is preferably formed with a
thickness equal to or larger than 200 .ANG., in order to form a
continuous film. Also, as shown in FIG. 25, a protective layer 27
may be provided between the substrate 1 and the first magnetic
layer 2, and said protective layer 47 is also preferably formed
with a thickness equal to or larger than 200 .ANG., in order to
obtain a continuous film. The corrosion resistance of the medium
can be further improved by adhering another substrate 49, as shown
in FIG. 26, onto the protective layer 46 by means of an adhesive
layer 48. Furthermore, recording and reproduction can be made from
both sides if the layers 47 to 46 are formed also on the substrate
49.
Said protective layer is composed of a dense non-magnetic material,
usually an inorganic dielectric material, such as Si.sub.3 N.sub.4,
SiC, ZnS, AlN, SiO, Al.sub.2 O.sub.3, Si or Ge. In the structures
shown in FIGS. 25 and 26, the protective layers 46, 47 may be
composed of different materials, but are preferably formed with a
same material in consideration of the stability of the magnetic
characteristics during storage. In FIGS. 24 to 26, same components
as those in FIG. 3 are represented by same numbers or symbols, and
will not be explained further.
EXAMPLE 25
A pregrooved and preformatted glass disk substrate was set in a
sputtering apparatus with ternary targets, and was rotated at a
distance of 10 cm from the targets.
A GdTbFeAl alloy was sputtered, in argon gas, from a second target
with a sputtering speed of 100 .ANG./min. and a sputtering pressure
of 5.times.10.sup.-3 Torr to obtain a first magnetic layer of
Gd.sub.15 Tb.sub.5 Fe.sub.79 Al.sub.1 of a thickenss of 210 .ANG.,
T.sub.L =ca. 165.degree. C. and H.sub.H =ca. 10 KOe.
Then a TbFeCoCr alloy was sputtered, in argon gas, with a
sputtering pressure 5.times.10.sup.-3 Torr to obtain a second
magnetic layer of Tb.sub.23 Fe.sub.60 Co.sub.14 Cr.sub.3 with a
thickness of 500 .ANG., T.sub.H =ca. 190.degree. C. and H.sub.L
=ca. 1 KOe.
Subsequently a ZnS protective layer of 1500 .ANG. in thickness was
formed by sputtering, in argon gas, from a first target with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After these layer formations, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. Said disk was mounted on a
record/reproducing apparatus, and was made to pass through, with a
linear speed of ca. 5 m/sec., a unit generating a magnetic field of
2.5 KOe. Then the recording was conducted with a laser beam of a
wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in
two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency
of 500 KHz. The bias field was 100 Oe. Binary signals could then be
reproduced by irradiation with a laser beam of 1.0 mW.
The above-explained experiment was repeated with a magnetooptical
disk recorded already over the entire surface. The previously
recorded signal components were not detected, and the possibility
of over-writing was thus confirmed.
EXAMPLE 26
Samples 26-1 to 26-11 of the structures shown in Tab. 11 were
prepared in the same manner as in the Example 25. In Tab. 12,
Mag(1) and Mag(2) indicate the first and second magnetic layers
prepared as explained in the Example 25 and composed respectively
of Gd.sub.15 Tb.sub.5 Fe.sub.79 Al.sub.1 and Tb.sub.23 Fe.sub.60
Co.sub.14 Cr.sub.3 with same thicknesses and coercive forces
explained before. Also glass means a glass disk substrate. Tab. 12
shows the values of H.sub.L and .sigma..sub.w /2M.sub.s h measured
on each sample.
TABLE 12 ______________________________________ Protective layer-
H.sub.L Sample Structure thickness (Oe) .sigma..sub.w /2M.sub.s h
______________________________________ 26-1 glass/Mag (1)/Mag (2)
-- 1000 800 26-2 glass/Mag (1)/Mag (2)/ ZnS-100 .ANG. 1000 400
Protect. layer 26-3 glass/Mag (1)/Mag (2)/ ZnS-200 1000 350
Protect. layer 26-4 glass/Mag (1)/Mag (2)/ ZnS-400 1000 300
Protect. layer 26-5 glass/Mag (1)/Mag (2)/ ZnS-1000 1000 300
Protect. layer 26-6 glass/Mag (1)/Mag (2)/ Si.sub.3 N.sub.4 -200
1000 300 Protect. layer 26-7 glass/Mag (1)/Mag (2)/ SiC-200 1000
300 Protect. layer 26-8 glass/Mag (1)/Mag (2)/ SiO-300 1000 300
Protect. layer 26-9 glass/Mag (1)/Mag (2)/ Si.sub.3 N.sub.4 -1000
1000 300 Protect. layer 26-10 glass/Mag (1)/Mag (2)/ SiC-1000 1000
300 Protect. layer 26-11 glass/Mag (1)/Mag (2)/ SiO-1000 1000 300
Protect. layer ______________________________________
The values of H.sub.L and .sigma..sub.w /2M.sub.s h were measured
by preparing a sample, composed of a first magnetic layer and a
second magnetic layer on a substrate, in the same process as in the
Example 25, and determining the magnetic field inducing the
inversion of the magnetization of the second magnetic layer, by
means of the magnetooptical effect and in the presence of an
external magnetic field.
In the following there will be explained the relation, examined
from the results shown in Tab. 12, between the protective layer and
the condition H.sub.L >.sigma..sub.w /2M.sub.s h for the
stability of the record bit 4f shown in FIG. 5.
The sample 26-1 did not have a protective layer in contact with the
second magnetic layer, and H.sub.L -.sigma..sub.w /2M.sub.s h was
200 Oe. This indicates that the mutually opposite magnetizations of
the first and second magnetic layers can stably exist with a margin
of 200 Oe in the absence of the external field. In practice,
however, 1-5% of the magnetization of the second magnetic layer is
inverted because inverted domains are generated by a smaller
field.
The samples 26-2 to 26-5, having a ZnS protective layer in contact
with the second magnetic layer, showed a decrease in .sigma..sub.w
/2M.sub.s h with the increase in thickness of said protective
layer. At a thickness of ZnS layer between 100 and 200 .ANG.,
.sigma..sub.w /2M.sub.s h reaches a substantially constant value
same as that at a ZnS thickness of 1000 .ANG., providing a margin
of ca. 700 Oe. These samples did not show the above-mentioned
inversion of magnetization of the second magnetic layer in the
absence of the external field, and were therefore stable.
Electron microscope observation of the samples proved that the
protective layer was transformed from an island structure to a
continuous film at a thickness of 100-200 .ANG.. It is estimated
that the protective layer generates a compression stress in the
second magnetic layer, thus inducing a change in .sigma..sub.w
/2M.sub.s h.
In the samples 26-2 to 26-11 investigated was the effect of
material constituting the protective layer formed in contact with
the second magnetic layer. It was confirmed that a protective layer
composed of Si.sub.3 N.sub.4, SiC or SiO had an effect of reducing
.sigma..sub.w /2M.sub.s h, or of stabilizing the record bit.
When the samples 26-2 to 26-11 were maintained in an atmosphere of
a temperature 45.degree. C. and a humidity of 65% for 3 days, the
sample 26-2 with the protective layer of 100 .ANG.showed an
increase of by about 30%, other samples showed decreases less than
10%.
EXAMPLE 27
A progrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus, with ternary targets, and was rotated at
a distance of 10 cm from the targets.
A Si protective layer of 500 .ANG. in thickness was formed by
sputtering, in argon gas, from a first target with a sputtering
speed of 100 .ANG./min and a sputtering pressure of
5.times.10.sup.-3 Torr. Then a GdTbFeCo alloy was sputtered, in
argon gas, from a second target with a sputtering speed of 100
.ANG./min. and a sputtering pressure of 5.times.10.sup.-3 Torr to
obtain a first magnetic layer of Gd.sub.10 Tb.sub.10 Fe.sub.78
Co.sub.2 with a thickness of 190 .ANG., T.sub.L =ca. 160.degree. C.
and H.sub.H =ca. 12 KOe.
Then a TbDyFeCo alloy was sputtered, in argon gas, with a
sputtering pressure of 5.times.10.sup.-3 Torr to obtain a second
magnetic layer of Tb.sub.12 Dy.sub.12 Fe.sub.64 Co.sub.12 of a
thickness of 500 .ANG., T.sub.H =ca. 185.degree. C. and H.sub.L
=ca. 1 KOe.
Then a Si protective layer of 1000 .ANG. in thickness was formed by
sputtering, in argon gas, from the first target with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After these layer formations, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
botain a magnetooptical disk. Said disk was mounted on a
record/reproducing apparatus and was made to pass through, with a
linear speed of ca. 8 m/sec., a unit generating a magnetic field of
2.5 KOe. The recording operation was then conducted with a laser
beam of a wavelength of 830 nm, concentrated to ca. 1 .mu.m and
modulated at two levels of 4 and 8 mW with a duty ratio of 50% and
a frequency of 2 MHz. The bias field was 100 Oe in a direction to
facilitate the inversion of magnetization of the second magnetic
layer. Then binary signals could be reproduced by irradiation with
a laser beam of 1.5 mW.
The above-explained experiment was repeated with a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
so that the possibility of overwriting was confirmed.
EXAMPLE 28
Samples 28-1 to 28-16 were prepared in the same manner as in the
Example 27, as shown in Tab. 13. The first magnetic layer (Mag(1))
and the second magnetic layer (Mag(2)) were respectively composed
of Gd.sub.10 Tb.sub.10 Fe.sub.78 Co.sub.2 and Tb.sub.12 Dy.sub.12
Fe.sub.64 Co.sub.12 of the same thicknesses and coercive forces as
in the Example 27. PC indicates a polycarbonate substrate.
Tab. 13 shows the values of H.sub.L and .sigma..sub.w /2M.sub.s h
measured on said samples.
TABLE 13
__________________________________________________________________________
Prot. layer (1) Prot. layer (2) material/ material/ .sigma..sub.w
/2M.sub.s h .sigma..sub.w /2M.sub.s h (Oe) Sample Structure
thickness (.ANG.) thickness (.ANG.) H.sub.L (Oe) (Oe) after storage
__________________________________________________________________________
28-1 PC/Prot. Layer (1)/ ZnS/150 -- 1200 1000 -- Mag (1)/Mag (2)
28-2 PC/Prot. Layer (1)/ ZnS/1000 -- 1000 800 -- Mag (1)/Mag (2)
28-3 PC/Prot. Layer (1)/ ZnS/800 ZnS/100 1000 400 -- Mag (1)/Mag
(2)/ Prot. Layer (2) 28-4 PC/Prot. Layer (1)/ ZnS/800 ZnS/200 1000
350 700 Mag (1)/Mag (2)/ Prot. Layer (2) 28-5 PC/Prot. Layer (1)/
ZnS/800 ZnS/400 1000 300 300 Mag (1)/Mag (2)/ Prot. Layer (2) 28-6
PC/Prot. Layer (1)/ ZnS/800 ZnS/800 1000 300 300 Mag (1)/Mag (2)/
Prot. Layer (2) 28-7 PC/Prot. Layer (1)/ SiO/800 SiO/800 1000 300
300 Mag (1)/Mag (2)/ Prot. Layer (2) 28-8 PC/Prot. Layer (1)/
SiO/800 SiO/800 1000 300 450 Mag (1)/Mag (2)/ Prot. Layer (2) 28-9
PC/Prot. Layer (1)/ Si.sub.3 N.sub.4 /800 Si.sub.3 N.sub.4 /800
1000 300 300 Mag (1)/Mag (2)/ Prot. Layer (2) 28-10 PC/Prot. Layer
(1)/ SiC/800 SiC/800 1000 300 300 Mag (1)/Mag (2)/ Prot. Layer (2)
28-11 PC/Prot. Layer (1)/ ZnS/800 SiO/800 1000 300 400 Mag (1)/Mag
(2)/ Prot. Layer (2) 28-12 PC/Prot. Layer (1)/ ZnS/800 SiC/800 1000
300 450 Mag (1)/Mag (2)/ Prot. Layer (2) 28-13 PC/Prot. Layer (1)/
ZnS/800 Si.sub.3 N.sub.4 /800 1000 300 500 Mag (1)/Mag (2)/ Prot.
Layer (2) 28-14 PC/Prot. Layer (1)/ Si.sub.3 N.sub.4 /800 ZnS/800
1000 300 500 Mag (1)/Mag (2)/ Prot. Layer (2) 28-15 PC/Prot. Layer
(1)/ Si.sub.3 N.sub.4 /800 SiC/800 1000 300 450 Mag (1)/Mag (2)/
Prot. Layer (2) 28-16 PC/Prot. Layer (1)/ Si.sub.3 N.sub.4 /800
SiO/800 1000 300 450 Mag (1)/Mag (2)/ Prot. Layer (2)
__________________________________________________________________________
The values of H.sub.L and .sigma..sub.w /2M.sub.s h were measured
by preparing a sample, composed of a first magnetic layer and a
second magnetic layer on a substrate, formed in the same manner as
in the Example 27, and determining the magnetic field inducing the
inversion of magnetization of the second magnetic layer, by means
of the magnetooptical effect, in the presence of an external
magnetic field.
The value of .sigma..sub.w /2M.sub.s h after storage was obtained
by placing the samples 28-1 to 28-16 in an atmosphere of a
temperature of 60.degree. C. and a relative humidity of 80% for 500
hours. No results are shown for the samples 28-1 to 28-3, as the
measurement was not possible by the deterioration of the magnetic
characteristic of the second magnetic layer.
In the following there will be explained the relation, examined
from the results shown in Tab. 13, between the protective layers
provided on both sides of the first and second magnetic layers, and
the condition H.sub.L >.sigma..sub.w /2M.sub.s h for the
stability of the record bit 4f shown in FIG. 5.
The samples 28-1, 28-2 did not have a protective layer in contact
with the second magnetic layer, and H.sub.L -.sigma..sub.w
/2M.sub.s h=200 Oe. This signifies that the mutually unstable
arrangement of magnetizations of the first and second magnetic
layers can stably exist with a margin of 200 Oe, in the absence of
the external field. In PG,119 practice, however, 1-5% of the
magnetization of the second magnetic layer is inverted because
inverted domains are generated by a smaller field.
The samples 28-3 to 28-6, having ZnS protective layers in contact
with the first and second magnetic layers, showed a decrease in
.sigma..sub.w /2M.sub.s h with the increase in the thickness of
said protective layers. At a thickness of ZnS layers above ca. 200
.ANG., .sigma..sub.w /2M.sub.s h reaches a substantially constant
value, providing a margin of ca. 700 Oe. These samples did not show
the above-mentioned inversion of magnetization of the second
magnetic layer in the absence of the external field, and the record
bits were therefore stable.
Electron microscope observation of the samples proved that the
protective layer was transformed from an island structure to a
continuous film at a thickness of 100-200 .ANG.. It is estimated
that the protective layer generates a compression stress in the
second magnetic layer, thus inducing a change in .sigma..sub.w
/2M.sub.s h.
In the samples 28-7 to 28-16, the protective layers were fixed to a
thickness of 800 .ANG. at which the effect on .sigma..sub.w
/2M.sub.s h is already saturated and which provides a sufficient
protective effect for example for preventing the entry of moisture
into the magnetic layers.
In the sample 28-7 the sputtering pressure for SiO was
3.times.10.sup.-3 Torr for both first and second layers, while in
the sample 28-8 the sputtering pressure for SiO was
8.times.10.sup.-3 Torr for the second layer only. The sample 28-7
is a case in which the protective layers have a same stress, while
the sample 28-8 is a case in which the protective layers have
mutually different stresses.
As a reference, the magnitude of stress was measured on samples for
stress measurement, prepared by forming a SiO layer of ca. 5000
.ANG. by sputtering on a glass substrate of 1.5 mm in thickness.
The layer formed with a sputtering pressure of 3.times.10.sup.-3
Torr provided a compression stress of ca. 70 kg/mm.sup.2, while
that formed with a pressure of 8.times.10.sup.-3 Torr provided a
compression stress of ca. 15 kg/mm.sup.2.
The samples 28-7 and 28-8 showed a value of .sigma..sub.w /2M.sub.s
h of ca. 300 Oe immediately after the preparation.
However, after a storage of 500 hours at 60.degree. C. and 80%
relative humidity, the sample 28-8 with two SiO layers of different
compression stresses showed an increase in .sigma..sub.w /2M.sub.s
h, rendering the record bits more unstable.
The samples 28-9 to 28-16 were prepared with various materials for
the protective layers, but the sputtering pressures therefor were
kept constant at 5.times.10.sup.-3 Torr.
All the samples showed a value of .sigma..sub.w /2M.sub.s h of ca.
300 Oe immediately after the preparation. Thus the recording is
made stabler compared to the case without the protective
layers.
However, after a storage for 500 hours at 60.degree. C. and 80%
relative humidity, the samples in which two protective layers are
composed of different materials showed a change in .sigma..sub.w
/2M.sub.s h.
The ZnS, SiC or Si.sub.3 N.sub.4 sputtered layer of 5000 .ANG. in
thickness, when subjected to the measurement of stress in the same
manner as the SiO layer, showed a compression stress of ca. 50, 80
or 90 kg/mm.sup.2 respectively. When stored for 500 hours at
60.degree. C. and 80% relative humidity, these samples showed an
increase in the stress, which was as high as ca. 20 to 30% in SiC
and Si.sub.3 N.sub.4.
These results indicate that the stability of record bits is related
to the stress of the protective layers. Particularly in
consideration of prolonged storage, the balance of stresses of the
protective layers should remain unchanged, and, for this purpose it
is effective to prepare both protective layers with a same
material, and with same manufacturing conditions such as the
sputtering pressure.
Triple-layered Structure
The magnetooptical recording medium of the present invention may
have a triple-layered structure, by adding a magnetic layer to the
above-explained two-layered structure. Such triple-layered
structure will be explained further in the following.
FIGS. 27 and 28 are schematic cross-sectional views of embodiments
of the magnetooptical recording medium of the present invention.
The medium shown in FIG. 27 is composed of a pregrooved translucent
substrate 51, and, a first magnetic layer 52, a second magnetic
layer 53 and a third magnetic layer 54 laminated thereon. The first
magnetic layer 52 has a high Curie point (T.sub.H1) and a low
coercive force (H.sub.L1), while the second magnetic layer has a
low Curie point (T.sub.L2) and a high coercive force (H.sub.H2),
and the third magnetic layer 54 has a high Curie point (T.sub.H3)
and a low coercive force (H.sub.L3). The terms "high" and "low" are
defined by relative comparison between the first and third magnetic
layers and the second magnetic layer, the comparison of coercive
force being at room temperature.
Relation of magnitude of the Curie point or the coercive is not
critically defined between the first and the third magnetic layers,
but preferred relations are T.sub.H1 .gtoreq.T.sub.H3 and H.sub.L1
.gtoreq.H.sub.L3.
It is generally desirable that the first magnetic layer 52 has
T.sub.H1 of 150.degree.-400.degree. C. and H.sub.L1 of 0.1-1 KOe,
the second magnetic layer 53 has T.sub.L2 of 70.degree.-200.degree.
C. and H.sub.H2 of 2-10 KOe, and the third magnetic layer 54 has
T.sub.H3 of 100.degree.-250.degree. C. and H.sub.L3 of 0.5-4
KOe.
In the magnetooptical recording medium of the present invention,
the neighboring magnetic layers are coupled by exchange force. The
first and second magnetic layers are relatively strongly coupled,
while the second and third are relatively weakly coupled.
In the magnetooptical recording medium of the present invention,
the above-mentioned three magnetic layers are so coupled as to
satisfy following relations: ##EQU7## wherein: .sigma..sub.w12 :
magnetic wall energy of first and second magnetic layers 52,
53;
.sigma..sub.w23 : magnetic wall energy of second and third magnetic
layers 53, 54;
h.sub.1, h.sub.2, h.sub.3 thicknesses of first, second and third
magnetic layers 52, 53, 54;
M.sub.s1, M.sub.s2, M.sub.s3 : saturation magnetizations of first,
second and third magnetic layers.
The reason of these relations will be explained later.
The thickness, coercive force, saturation magnetization and
magnetic wall energy of said magnetic layers 52, 53, 54 are so
selected that the two states of magnetization (shown by 60f in FIG.
29) of the finally recorded bits can exist in stable manner, or
that the above-mentioned relations are satisfied.
Each magnetic layer can be composed of a substance exhibiting a
vertical magnetic anisotropy and a magnetooptical effect,
preferably an amorphous magnetic alloy of a rare earth element and
a transition metal element such as GdCo, GdFe, TbFe, DyFe, GdTbFe,
TbDyFe, GdFeCo, TbFeCo or GdTbCo.
FIG. 28 shows another embodiment of the magnetooptical disk of the
present invention, in which protective layers 55, 56 are provided
for improving the durability or enhancing the magnetooptical effect
of the three magnetic layers 52, 53, 54.
An adhesive layer 57 is provided for adhering another covering
substrate 58. Also recording and reproduction can be made from both
sides if the covering substrate 58 with the layers from 55 to
56.
Now reference is made to FIGS. 29 and 30 for explaining the
recording process of the present embodiment, wherein, prior to the
recording, the magnetization of the magnetic layer 52 and that of
the magnetic layer 53 may be in a mutually parallel stable state,
or in a mutually opposite (antiparallel) stable state.
The aforementioned apparatus shown in FIG. 6 can also be employed
in the recording and reproduction of information to or from the
magnetooptical recording medium of the present embodiment. It is
assumed that a part of the magnetic layers has an initial
magnetization as shown by 60a in FIG. 29. More specifically it is
assumed, in this case, that the magnetizations of the first, second
and third magnetic layers are stable, prior to recording, when they
are oriented in a same direction. The magnetooptical disk 9, being
rotated by a spindle motor, passes the position of a magnetic field
generating unit 8 shown in FIG. 6, generating a magnetic field of
which intensity is selected at a suitable level between the
coercive forces of the second and third magnetic layers 53, 54
(magnetic field being upwards in the present embodiment), whereby,
as shown by 60d in FIG. 29, the third magnetic layer 54 is
uniformly magnetized while the second magnetic layer 53 retains the
initial magnetization state. Also the first magnetic layer 52,
strongly coupled with the second magnetic layer, retains the
initial magnetization.
The rotated magnetooptical disk 9, in passing the position of a
record/reproducing head 5, is irradiated by a laser beam with one
of two power levels according to the signal from a recording signal
generator 6. The first laser power is sufficient to heat the disk
to a temperature close to the Curie point of the second magnetic
layer 53, while the second laser power is enough for heating the
disk to a temperature close to the Curie point of the third
magnetic layer 54. Referring to FIG. 30 showing the temperature
dependence of the coercive forces of the magnetic layers 53, 54,
the first laser power can heat the disk close to T.sub.L2, while
the second laser power can heat the disk close to T.sub.H3.
The first laser power heats the second and third magnetic layers
53, 54 close to the Curie point of the second magnetic layer 53,
but the third magnetic layer 54 has a coercive force capable of
stably maintaining the bit at this temperature. Thus, through a
suitable selection of the recording bias magnetic field, a record
bit shown by 60c in FIG. 29 can be obtained, as a preliminary
recording of first type, from either state in 60b. The first
magnetic layer 52 assumes the illustrated magnetization by the
exchange coupling with the second magnetic layer 53.
The suitable selection of the bias magnetic field has the following
meaning.
In said preliminary recording of first type, such bias magnetic
field is essentially unnecessary, since the second magnetic layer
53 receives a force (exchange force) to orient the magnetization in
a stable direction (same direction in this case) with respect to
the direction of magnetization of the third magnetic layer 54.
However said bias magnetic field is provided, in a preliminary
recording of second type to be explained later, in a direction to
assist the magnetic inversion of the third magnetic layer 54. Also
said bias magnetic field should preferably have a same magnitude
and a same direction in the preliminary recording with both first
laser power and second laser power. In consideration of the
foregoing, the bias magnetic field is preferably selected at a
minimum necessary intensity required for the preliminary recording
with the second laser power of the principle explained below.
When the disk is heated close to the Curie point of the third
magnetic layer by a second laser power (preliminary recording of
second type), the magnetization of said third magnetic layer 54 is
inverted by the above-mentioned bias magnetic field. Also the
magnetizations of the second and first magnetic layers 53, 52 are
oriented in a stable direction (same direction in this case) with
respect to the magnetization of the third magnetic layer 54. In
this manner a record bit as shown by 60b, in FIG. 29, can be from
either state 60b.
Thus each area of the magnetooptical disk can have a record state
60c or 60d, shown in FIG. 29, by the bias field and by the first or
second laser power corresponding to the input signal.
Then the magnetooptical disk 9 is further rotated, so that the
record bit 60c or 60d passes again through the magnetic field
generating unit 8, of which field intensity is selected, as
explained before, between the coercive forces of the second and
third magnetic layers 53, 54. Thus the record bit 60c remains
unchanged and assumes a state 60e. On the other hand, the record
bit 60d causes the inversion of magnetization of the third magnetic
layer 54 to assume a state 60f.
In order that the record bit 60f can exist stably, there are
required the aforementioned relations: ##EQU8## for the following
reasons.
.sigma..sub.w12 /2M.sub.s1 h.sub.1 indicates the magnitude of the
exchange force received by the first magnetic layer, or represents
the magnitude of a magnetic field acting to orient the
magnetization of the first magnetic layer in a stable direction
(same direction in the present case) with respect to the
magnetization of the second magnetic layer. Therefore, in order
that the magnetization of the first magnetic layer is always
oriented in a stable direction (same direction in the present case)
with respect to the magnetization of the second magnetic layer, the
coercive force H.sub.L1 of the first magnetic layer should be
smaller than said exhange force, or .sigma..sub.w12 /2M.sub.s1
h.sub.1 >H.sub.L1.
Also, .sigma..sub.w23 /2M.sub.s3 h.sub.3 indicates the magnitude of
the exchange force received by the third magnetic layer, or
represents the magnitude of a magnetic field acting to orient the
magnetization of the third magnetic layer in a stable direction
(same direction in the present case) with respect to the
magnetization of the second magnetic layer. In order that the
magnetization of the third magnetic layer is not inverted by said
magnetic field, or in order that the record 60f, in FIG. 29, can
stably exist, there should be satisfied a condition .sigma..sub.w23
/2M.sub.s3 h.sub.3 <H.sub.L3 wherein H.sub.L3 is the coercive
force of the third magnetic layer.
The second and third magnetic layers 53, 54 have to be
exchange-coupled in order to obtain an effective bias magnetic
field caused by the exchange force at the recording, but the
above-mentioned condition cannot be satisfied, and the record bits
60f cannot exist stably, if the exchange coupling is too strong. In
the preparation of the magnetooptical recording medium of the
present invention, therefore, it is possible to optimize said
exchange-coupling, by selecting the coercive force of the third
magnetic layer 54 at a relatively large value within a range not
exceeding the intensity of the magnetic field generated by the
magnetic field generating unit, and, if said exchange coupling is
excessively large, by suitably selecting the composition of the
third magnetic layer or by forming an intermediate layer of several
to several thousands of Angstroms in thickness between the second
and third magnetic layers (by exposing to gas or plasma reactive to
the material of the second magnetic layer after sputtering thereof
or by sputtering a dielectric layer from a target).
The recording process of the present embodiment enables an
overwriting operation, since the record bits 60e, 60f do not rely
on the state prior to recording but solely on the laser power at
the recording. The record bits 60e, 60f can be reproduced by
irradiation with a reproducing laser beam and processing the
resulting light with a signal reproducing unit 7 shown in FIG. 6.
The magnitude (modulation) of the reproduced signal principally
depends on the magnetooptical effect of the first magnetic layer.
This recording process can provide a recording with a reproduction
signal of a high magnitude or a high degree of modulation because
of the above-mentioned fact, and a fact that, in the medium of the
present invention having three magnetic layers, a material with a
high Curie point, or with a strong magnetooptical effect, can be
used for the first magnetic layer 52 receiving the reproducing
laser beam.
In the foregoing description relating to FIG. 29, it is assumed
that the magnetizations of the first, second and third magnetic
layers 52, 53, 54 are stable when they are oriented in a same
direction, but a similar process can be realized when the
magnetizations are stable when they are oppositely oriented. FIGS.
31 and 32 illustrate states of magnetization in such recording
process, wherein 61a-61f and 62a-62f respectively correspond to the
states 60a-60f in FIG. 29, and the recording is conducted in a
similar manner as shown in FIG. 29.
EXAMPLE 29
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with quaternary targets, and was rotated
at a distance of 10 cm from the targets.
A ZnS protective layer of 800 .ANG. in thickness was formed by
sputtering, in argon gas, from a first target with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
Then a GdFeCo alloy was sputtered, in argon gas, from a second
target with a sputtering speed of 100 .ANG./min. and a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer
of Gd.sub.20 Fe.sub.56 Co.sub.24 with a thickness of 400 .ANG. and
T.sub.H1 =ca. 350.degree. C. H.sub.L1 of said first magnetic layer
itself was lower than ca. 500 Oe, and the transition metal was
prevailing in the sub-lattice magnetization.
Then a TbFe alloy was sputtered from a third target under similar
conditions to obtain a second magnetic layer of Tb.sub.18 Fe.sub.82
with a thickness of 400 .ANG. and T.sub.L2 =ca. 140.degree. C.
H.sub.H2 of said second magnetic layer itself was above ca. 5000
Oe, and the transition metal was prevailing in the sub-lattice
magnetization.
Then a GdTbFeCo alloy was sputtered from a fourth target under
similar conditions to obtain a third magnetic layer of Gd.sub.13
Tb.sub.13 Fe.sub.69.5 Co.sub.4.5 with a thickness of 300 .ANG. and
T.sub.H3 =ca. 210.degree. C. H.sub.L3 of said third magnetic layer
itself was ca. 500-1500 Oe, and the rare earth metal was prevailing
in the sub-lattice magnetization.
Subsequently a ZnS protective layer of 2000 .ANG. in thickness was
formed by sputtering from the first target.
After these layer formations, the above mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. Said disk was mounted on a
record/reproducing apparatus, and was made to pass through, with a
linear speed of ca. 7 m/min., a unit generating a magnetic field of
2 KOe. Recording was then conducted with a laser beam of a
wavelength of 830 nm, concentrated to ca. 1 .mu.m and modulated in
two levels of 4 and 8 mW, with a duty ratio of 50% and a frequency
of 2 MHz. The bias field was 150 Oe. Binary signals could be then
reproduced by irradiation with a laser beam of 1 mW.
Also recording test was made in the same method as in the Example
29, and the state of record of 4 and 8 mW was evaluated, as shown
in Tab. 14.
TABLE 14
__________________________________________________________________________
Composition Recording bit 1st mag. layer 2nd mag. layer 3rd mag.
layer stability Record state Sample rich in rich in rich in 60e 60f
4 mW 8 mW
__________________________________________________________________________
Ex. 29 trans. metal trans. metal rare earth + + + + 30-1 rare earth
rare earth trans. metal + + + + 30-2 trans. metal trans. metal
trans. metal + - - .+-. 30-3 trans. metal rare earth rare earth + -
- .+-. 30-4 rare earth trans. metal trans. metal + - - .+-. 30-5
rare earth rare earth rare earth + - - .+-. 30-6 rare earth trans.
metal rare earth .+-. .+-. .+-. .+-. 30-7 trans. metal rare earth
trans. metal .+-. .+-. .+-. .+-.
__________________________________________________________________________
The stability of record bits is represented by "+" or "-"
respectively if the state 60e or 60f can stably exist or not in the
absence of the external magnetic field.
The record state was evaluated as "-" if the reproduction signal
could not be confirmed from the record with 4 mW or 8 mW, ".+-." if
the reproduction signal could be confirmed but not satisfactory,
and "+" if a satisfactory reproduction signal of C/N ratio of about
40 dB or higher could be obtained.
The results in Tab. 14 indicates that, for achieving satisfactory
recording with stable record bits, the first and second magnetic
layers should preferably be rich simultaneously in the transition
metal (transition metal prevailing in the sub-lattice
magnetization) or in the rare earth element (rare earth element
prevailing in the sub-lattice magnetization) compared to the
compensation composition, and the third magnetic layer should
preferably prevailing element opposite to that in the first and
second magnetic layers, as in the samples 29 and 30-1.
In another embodiment of the triple-layered magnetooptical
recording medium of the structure shown in FIGS. 27 and 28, thee
may be adopted relations:
wherein T.sub.1, T.sub.2 and T.sub.3 are the Curie points
respectively of the first, second and third magnetic layers 52, 53,
54, and H.sub.1, H.sub.2 and H.sub.3 are the coercive forces
respectively of said magnetic layers.
In general, the first magnetic layer 52 has T.sub.1 in a range from
70.degree. to 200.degree. C. and H.sub.1 in a range from 2 to 10
KOe, while the second magnetic layer 53 has T.sub.2 from 90.degree.
to 400.degree. C. and H.sub.2 from 0.1 to 1 KOe, and the third
magnetic layer 54 has T.sub.3 from 150.degree. to 400.degree. C.
and H.sub.3 from 0.5 to 4 KOe. In such case, in order to achieve
stable recording, the layer thicknesses, saturation magnetizations
and magnetic wall energies should preferably be so regulated as to
substantially satisfy following relations: ##EQU9## wherein
.sigma..sub.w12 is the magnetic wall energy of the first and second
magnetic layers; .sigma..sub.w23 is the magnetic wall energy of the
second and third magnetic layers; h.sub.1, h.sub.2 and h.sub.3 are
the thicknesses of the first, second and third magnetic layers; and
M.sub.s1, M.sub.s2 and M.sub.s3 are the saturation magnetizations
of said layers.
The information recording is conducted in the following manner,
with the apparatus shown in FIG. 6.
It is assumed, prior to the recording, that the magnetizations of
the first magnetic layer 52 and of the second magnetic layer 53 are
in a mutually opposite (antiparallel) stable state, while the
magnetizations of the second and third magnetic layers 53, 54 are
in a mutually parallel stable state. It is also assumed that a part
of the magnetic layers is in a state of magnetization shown by 63a
in FIG. 33.
The magnetooptical disk 9, being rotated by the spindle motor as
shown in FIG. 6, passes through the position of the magnetic field
generating unit 8, generating a magnetic field, which is upwards in
the present case and of which intensity is adjusted to a value
between the coercive forces of the first and second magnetic layers
52, 53, whereby the second and third magnetic layers 53, 54 are
uniformly magnetized but the first magnetic layer 52 retains the
initial state of magnetization, as shown by 63b in FIG. 33.
Then the magnetooptical disk 9 is rotated, and, in passing the
record/reproducing head 5, is irradiated by a laser beam with one
of two power levels, according to a signal from the recording
signal generator 6. The first laser power is enough for heating the
disk to a temperature close to the Curie point of the first
magnetic layer (a temperature close to T.sub.1 and allowing to
orient the magnetization of the first magnetic layer in a stable
direction with respect to the magnetizations of the second and
third magnetic layers), while the second laser power is enough for
heating the disk to a temperature close to the Curie points of the
second and third magnetic layers (a temperature close to T.sub.2
and T.sub.3 and allowing to uniformly invert the magnetizations of
the second and third magnetic layers). Referring to FIG. 34 showing
the temperature dependence of the coercive forces of the magnetic
layers 52, 54, the first laser power can elevate the temperature of
the disk close to T.sub.1 while the second laser power can elevate
the temperature of the disk close to T.sub.3.
The first laser power heats the first and third magnetic layers 52,
54 close to the Curie point of the first magnetic layer 52, but the
third magnetic layer 54 has a coercive force allowing to stably
maintain the record bit at this temperature. Thus, through a
suitable selection of the bias magnetic field, a record bit as
shown by 63c in FIG. 33 can be formed, as a preliminary recording
of first type, from either state of magnetization shown by 63b. The
magnetization of the second magnetic layer, having a stronger
exchange force with the third magnetic layer than with the first
magnetic layer, becomes always parallel with the magnetization of
the third magnetic layer, as illustrated.
The suitable selection of the bias magnetic field has the following
meaning.
In said preliminary recording of first type, the bias magnetic
field is essentially unnecessary because the first magnetic layer
52 receives an exchange force to orient the magnetization thereof
in a stable (opposite or antiparallel in the present case)
direction with respect to the magnetization of the third magnetic
layer 54. However the bias magnetic field is provided for the
preliminary recording with the second laser power to be explained
later, in a direction to assist the inversion of magnetization of
the third magnetic layer 54, or a direction to hinder the
preliminary recording of first type. It is convenient that said
bias magnetic field is maintained in a same intensity and in a same
direction, both in the preliminary recording of first and second
types. In consideration of these conditions, the bias magnetic
field should preferably be maintained at a minimum necessary level
for the preliminary recording of second type to be explained in the
following, and such bias field corresponds to the above-mentioned
suitable selection.
When the disk is heated close to the Curie point of the third
magnetic layer 54 by the second laser power for the preliminary
recording of second type, the magnetizations of the third and
second magnetic layers 54, 53 are inverted by the bias magnetic
field selected as explained above. Subsequently the magnetization
of the first magnetic layer 52 is also oriented in a stable
(opposite or antiparallel in the present case) direction with
respect to the magnetization of the third magnetic layer 54. In
this manner a preliminary recording bit as shown by 63d in FIG. 33
can be formed from either state of magnetization shown by 63b.
In this manner each area of the magnetooptical disk can form a
preliminary record of the state 63c of 63d as shown in FIG. 33, by
means of the bias magnetic field and the laser power of first or
second level according to the input signal.
Then the magnetooptical disk 9 is further rotated and passes again
the position of the magnetic field generating unit 8, generating a
magnetic field of which intensity is selected between the coercive
forces of the first and third magnetic layers 52, 54 as explained
before, whereby the record bit 63c remains unchanged as a final
record bit 63e, while the record bit 63d assumes another final
record state 63f as the result of magnetic inversion of the third
magnetic layer.
In order that the record bit 63f can stably exist, the
aforementioned conditions (13) to (15) should be satisfied, because
of the following reasons.
.sigma..sub.w12 /2M.sub.s1 h.sub.1 indicates the magnitude of the
exchange force received by the first magnetic layer, or represents
the magnitude of a magnetic field acting to rearrange the
magnetization of the first magnetic layer in a stable (opposite or
antiparallel in the present case) direction with respect to the
magnetization of the second magnetic layer. Therefore, in order
that the first magnetic layer can retain its magnetization
unchanged against said magnetic field, said layer should have a
coercive force H.sub.1 larger than the magnitude of said exchange
force (.sigma..sub.w12 /2M.sub.s1 h.sub.1 <H.sub.1.
The second magnetic layer 53 receives, from a magnetic wall at the
interface with the first magnetic layer 52, an exchange force of a
magnitude .sigma..sub.w12 /2M.sub.s2 h.sub.2 and of a direction to
orient the magnetization of the second magnetic layer in a stable
direction with respect to the magnetization of the first magnetic
layer 52, and also receives, from a magnetic wall at the interface
with the third magnetic layer 54, an exchange force of a magnitude
.sigma..sub.w23 /2M.sub.s2 h.sub.2 and of a direction to orient the
magnetization of the second magnetic layer in a stable direction
with respect to the magnetization of the third magnetic layer 54.
Consequently, in order that the magnetization of the second
magnetic layer is constantly oriented in a stable direction with
respect to the magnetization of the third magnetic layer during and
after the recording operation, there are required conditions
(.sigma..sub.w23 /2M.sub.s2 h.sub.2) >(.sigma..sub.w12
/2M.sub.s1 h.sub.1) and (.sigma..sub.w23 /2M.sub.s2
h.sub.2)>H.sub.2.
Also .sigma..sub.w23 /2M.sub.s3 h.sub.3 indicates the magnitude of
the exchange force received by the third magnetic layer 54, or
represents the magnitude of a magnetic field acting, across the
second magnetic layer 53, to rearrange the magnetization of the
third magnetic layer in a stable (opposite or antiparallel in the
present case) direction with respect to the magnetization of the
first magnetic layer 52. In order that the magnetization of the
third magnetic layer 54 is not inverted against said magnetic
field, or in order that the record bit 63f in FIG. 33 can stably
exist, the coercive force H.sub.3 of the third magnetic layer 54
should satisfy a condition (.sigma..sub.w23 /2M.sub.s3
h.sub.3)<H.sub.3.
From the foregoing explanation the interfacial magnetic wall
energies should satisfy a condition .sigma..sub.w23
>.sigma..sub.w12, so that:
a) it is desirable to realize a weak coupling between the first and
second magnetic layers, by adopting a composition rich in rare
earth element compared with the compensation composition for one of
said layers and a composition rich in transition metal for the
other of said layers; and
b) to realize a strong coupling between the second and third
magnetic layers by adopting compositions both rich in the rare
earth element or compositions both rich in the transition
metal.
Also it is possible to reduce the exchange force from the third
magnetic layer to the second magnetic layer, by reducing the
exchange force .sigma..sub.w12 /2M.sub.s2 h.sub.2 from the first
magnetic layer to the second magnetic layer, and the exchange force
.sigma..sub.w23 /2M.sub.s2 h.sub.2 from the third magnetic layer to
the second magnetic layer. For this purpose it is advantageous to
reduce the saturation magnetization M.sub.s2 of the second magnetic
layer, namely to minimize the coercive force H.sub.2 thereof.
Therefore at least a condition H.sub.3 >H.sub.2 is
desirable.
In the recording process of the present embodiment, the record bits
63e and 53f do not depend on the state prior to recording but are
solely controlled by the recording laser power, so that an
overwriting operation is possible. The record bits 63e, 63f can be
reproduced by irradiation with a reproducing laser beam and by
processing of the resulting light with the recording signal
reproducer 7.
EXAMPLE 31
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with four targets, and was rotated at a
distance of 10 cm from the targets.
A ZnS protective layer of 800 .ANG. in thickness was formed by
sputtering, in argon gas, from a first target, with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
Then a TbFe alloy was sputtered, in argon gas, from a second target
with a sputtering speed of 100 .ANG./min. and a sputtering pressure
of 5.times.10.sup.-3 Torr to obtain a first magnetic layer of
Tb.sub.18 Fe.sub.82 of a thickness of 300 .ANG. and T.sub.1 =ca.
140.degree. C. H.sub.1 of said first magnetic layer itself was ca.
10 KOe, and the transistion metal was prevailing in the sub-lattice
magnetization
Then a TbFeCo alloy was sputtered under similar conditions from a
third target to obtain a second magnetic layer of Tb.sub.25
Fe.sub.65 Co.sub.10 with a thickness of 150 .ANG. and T.sub.2 =ca.
210.degree. C. H.sub.2 of said second magnetic layer itself was
lower than ca. 200 Oe, and the rare earth element was prevailing in
the sub-lattice magnetization.
Subsequently a TbFeCo alloy was sputtered under similar conditions
from a fourthtarget to obtain a third magnetic layer of Tb.sub.23
Fe.sub.67 Co.sub.10 with a thickness of 300 .ANG.and T.sub.3 =ca.
210.degree. C. H.sub.L3 of said third magnetic layer itself was ca.
500-1500 Oe, and the rare earth element was prevailing in the
sub-lattice magnetization.
Then a ZnS protective layer of 2000 .ANG. in thickness was formed
by sputtering from the first target under similar conditions.
After said layer formation, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk. Said disk was mounted on a
record/reproducing apparatus, and was made to pass through, with a
linear speed of ca. 7 m/sec., a unit for generating a magnetic
field of 2 KOe. The recording was conducted with a laser beam of a
wavelength of 830 nm, concentrated to ca. 1 .mu. and modulated in
two levels of 4 and 8 mW with a duty ratio of 50% and a frequency
of 2 MHz. The bias field was 150 Oe. Then binary signals could be
reproduced by irradiation with a laser beam of 1 mW.
The above-explained experiment was repeated with a magnetooptical
disk already recorded over the entire surface. The previously
recorded signal components were not detected in the reproduction,
and the possibility of overwriting was thus confirmed.
EXAMPLE 32
Samples of magnetooptical disk shown in Tab. 15 were prepared in
the same manner as in Example 31. The thickness of the layers were
maintained same as in the Example 31 for the purpose of comparison,
but the compositions of the first, second and third magnetic layers
were selected in various combinations of compositions rich in the
rare earth element and those rich in the transition metal. The
coercive forces of the first and third magnetic layers were
respectively regulated to ca. 10 KOe and ca. 1 KOe through the
control of contents of Tb and Fe.
In Tab. 15, TM means a composition rich in the transition metal
compared with the compensation composition, and RE means a
composition rich in the rare earth element.
TABLE 15
__________________________________________________________________________
Composition 1st mag. 2nd mag. 3rd mag. layer layer layer H.sub.2 of
2nd Bit Sta- Record Sample rich in rich in rich in mag. layer
bility state
__________________________________________________________________________
EX. 31 TM RE RE 200 Oe + + 32-1 TM RE RE 500 + + 32-2 TM RE RE 1000
.+-. + 32-3 TM RE RE 1500 - - 32-4 RE TM TM 300 + + 32-5 RE TM TM
700 + + 32-6 TM TM RE 500 + - 32-7 TM TM TM 500 - - 32-8 RE RE TM
200 + -
__________________________________________________________________________
In the first magnetic layer, the TM-rich composition was Tb.sub.18
Fe.sub.82, while the RE-rich composition was Tb.sub.20 FE.sub.80.In
the third magnetic layer, the TM-rich composition was Tb.sub.15
FE.sub.75 Co.sub.10, while the RE-rich composition was Tb.sub.23
Fe.sub.67 Co.sub.10.
The stability of the record bit, particularly in the state 63f, was
investigated on thus prepared samples, by measuring the external
magnetic field inducing the inversion of magnetization by a VSM. In
the results shown in Tab. 15, "+" indicates stable existence of the
record bit in the state of 63f in the absence of external field,
while "-" indicates a partial inversion of the magnetization of the
third magnetic layer. Also the recordings of first and second types
were conducted in the same method as in the Example 31, and the
result was evaluated as "+" or "-" respectively if satisfactory
reproduction signal was obtained or not.
The results on the Example 31 and the Example 32-1 to 32-5 listed
in Tab. 15 indicate that satisfactory bit stability and
satisfactory record state could be obtained, as explained in the
foregoing, by a combination of a TM-rich first magnetic layer with
RE-rich second and third magnetic layers or of an RE-rich first
magnetic layer with TM-rich second and third magnetic layers, and
when the coercive force H.sub.2 of the second magnetic layer is
smaller than the coercive force H.sub.3 of the third magnetic
layer.
EXAMPLE 33
Samples of magnetooptical disk shown in Tab. 16 were prepared in
the same manner as in the Example 31. The thicknesses of the layers
were maintained same as in the Example 31 for the purpose of
comparison, but the composition of the second magnetic layer was
varied in comparison with the Example 31. The coercive force
thereof was less than ca. 200 Oe, and the rare earth element was
prevailing in the sub-lattice magnetization, as in the Example
31.
The prepared samples were evaluated in the same manner as in the
Example 32.
TABLE 16 ______________________________________ Composition of Bit
Record Sample 2nd mag. layer T.sub.2 (.degree.C.) stability state
______________________________________ 33-1 Tb.sub.24 Fe.sub.76 140
+ + 33-2 Tb.sub.13 Gd.sub.12 Fe.sub.75 160 + + 33-3 Tb.sub.13
Gd.sub.12 Fe.sub.73 Co.sub.2 180 + + 33-4 Tb.sub.13 Gd.sub.12
Fe.sub.71 Co.sub.4 200 + + 33-5 Dy.sub.25 Fe.sub.75 70 + .+-. 33-6
Tb.sub.5 Gd.sub.20 Fe.sub.70 Co.sub.d 250 + .+-. 33-7 Tb.sub.5
Gd.sub.20 Fe.sub.65 Co.sub.10 300 + -
______________________________________
The results of the samples listed in Tab. 16 indicate that
satisfactory recording could be obtained when the curie point
T.sub.2 of the second magnetic layer is located between T.sub.1 and
T.sub.3, (Examples 33-1 to 33-4), the recording of first type could
not be satisfactorily made if T.sub.2 was lower than T.sub.1
(Example 33-5).
Also the recording of second type could not be made satisfactorily
when T.sub.2 is higher than T.sub.3 (Examples 33-6, 33-7).
Quadruple-layered Structure
Furthermore, the magnetooptical recording medium of the present
invention may be provided with four magnetic layers, as will be
explained in the following.
FIGS. 35 and 36 are schematic cross-sectional views of
quadraple-layered embodiments of the magnetooptical recording
medium of the present invention. The medium shown in FIG. 35 is
composed of a pregrooved translucent substrate 71, and, a first
magnetic layer 72, a second magnetic layer 73, a third magnetic
layer 74 and a fourth magnetic layer 75 laminated thereon. Said
first, second, third and fourth magnetic layers respectively have
Curie points T.sub.1 T.sub.2, T.sub.3 and T.sub.4 ; coercive forces
H.sub.1, H.sub.2, H.sub.3 and H.sub.4 ; thicknesses h.sub.1,
h.sub.2, h.sub.3 and h.sub.4 ; and saturation magnetizations
M.sub.s1, M.sub.s2, M.sub.s3 and M.sub.s4. Also the magnetic wall
energies between the first and second magnetic layers, between the
second and third magnetic layers, and between the third and fourth
magnetic layers are respectively represented by .sigma..sub.w12,
.sigma..sub.w23 and .sigma..sub.w34. The above-mentioned four
magnetic layers satisfy following conditions I-IV, through mutual
exchange-couplings.
I. For the Curie points of the magnetic layers:
II. For the coercive forces of the magnetic layers:
III. For the thicknesses of the magnetic layers:
IV. or the saturation magnetizations, thicknesses, coercive forces
and magnetic wall energies of the magnetic layers: ##EQU10##
It is generally desirable that the first magnetic layer 72 has
T.sub.1 of 150.degree.-400.degree. C., H.sub.1 of 0.1-1 KOe, a
thickness h.sub.1 of 50-300 .ANG.; the second magnetic layer 73 has
T.sub.2 of 70.degree.-200.degree. C., H.sub.2 of 2-12 KOe, a
thickness h.sub.2 of 50-300 .ANG.; the third magnetic layer 74 has
T.sub.3 of 0.degree.-200.degree. C., H.sub.3 of 0.1-1 KOe, a
thickness h.sub.3 of 50-300 .ANG.; and the fourth magnetic layer 75
has T.sub.4 of 100.degree.-300.degree. C., H.sub.4 of 0.5-4 KOe,
and a thickness h.sub.4 of 50-600 .ANG..
Each magnetic layer can be principally composed of a substance
exhibiting a vertical magnetic anisotropy and a magnetooptical
effect, preferably an amorphous alloy of a rare earth element and a
transition metal element, such GdCo, GdFe, TbFe, DyFe, GdTbFe,
TbDyFe, GdTbFeCo, TbFeCo or GdTbCo.
In the magnetooptical recording medium of the present invention,
neighboring magnetic layers are coupled with an exchange force. The
first and second magnetic layers are relatively strongly couple,
while the second and third magnetic layers relatively weakly
coupled, and the third and fourth magnetic layers are relatively
weakly coupled. A large magnetic wall energy is present between the
strongly coupled layers, while a weak magnetic wall energy is
present between the weakly coupled layers. The value of the
magnetic wall energy .sigma..sub.w is optimized in regulating
M.sub.s, h, H and .sigma..sub.w for satisfying the aforementioned
relations IV.
In the four magnetic layers 72, 73, 74, 75 the thickness, coercive
force, saturation magnetization and magnetic wall energy of each
layer are so regulated to satisfy the aforementioned relations, and
such regulation will stabilize the finally completed two states of
magnetization shown by 70e and 70f in FIG. 37.
In another embodiment of the magnetooptical recording medium shown
in FIG. 36, protective layers 76, 77 are provided for improving the
durability of the four magnetic layers 72, 73, 74, 75 or enhancing
the magnetooptical effect thereof.
An adhesive layer 78 is provided for adhering a covering substrate
79. Recording and reproduction can be made from both sides if the
layers of 72 to 77 are formed also on said covering substrate
79.
Now reference is made to FIGS. 37 and 38 for explaining the
recording process of the present embodiment, wherein, prior to the
recording, among four magnetic layers, the magnetizations of
mutually neighboring layers may be in a mutually parallel stable
state, or in a mutually opposite (antiparallel) stable state.
However, since mutually parallel magnetizations provide a
relatively strong coupling while mutually antiparallel
magnetizations provide a relatively weak coupling, the
magnetooptical recording medium of the present invention should
preferably have such state of magnetization that the magnetizations
of the first and second magnetic layers are in a mutually parallel
stable state while those of the second and fourth magnetic layers
are in a mutually antiparallel stable state.
The recording and reproducing operation on the medium of the
present embodiment can also be conducted with the apparatus shown
in FIG. 6. It is assumed that a part of said medium has an initial
magnetization as shown by 70a in FIG. 37, and that the
magnetizations of the third and fourth magnetic layers are in a
mutually parallel stable state. As shown in FIG. 6, the
magnetooptical disk 9, being rotated by a spindle motor, passes the
position of a magnetic field generating unit 8, generating an
upward magnetic field of which intensity is selected at a suitable
level between the coercive forces of the second and fourth magnetic
layers 73, 75, whereby, as shown by 70b in FIG. 37, the fourth
magnetic layer 75 is uniformly magnetized, and the third magnetic
layer 74, strongly coupled with the fourth layer 75, is also
magnetized in the same direction. On the other hand, the second
magnetic layer 73 retains the initial magnetization, and the first
magnetic layer 72, strongly coupled with the second magnetic layer
73, also retains the initial magnetization.
The rotated magnetooptical disk 9, in passing the position of a
record/reproducing head 5, is irradiated by a laser beam with one
of two power levels according to the signal from a recording signal
generator 6. The first laser power is enough for heating the disk
to a temperature close to the Curie point of the second magnetic
layer 73, while the second laser power is enough for heating the
disk to a temperature close to the Curie point of the fourth
magnetic layer 75. Referring to FIG. 38 showing the temperature
dependence of the magnetic layers 73, 75, the first laser power can
heat the disk close to T.sub.2, while the second laser power can
heat the disk close to T.sub.4.
The first laser power heats the second and fourth magnetic layers
73, 75 close to the Curie point of the second magnetic layer, but
the fourth magnetic layer 75 has a coercive force capable of stably
maintaining the bit at this temperature. Thus, through a suitable
selection of the recording bias magnetic field, the magnetizations
of the magnetic layers are oriented in stable directions with
respect to the magnetization of the fourth magnetic layer in the
course of temperature lowering of the record bit area.
In this manner a record bit shown by 70c in FIG. 37 can be formed,
as a preliminary recording of first type, from either state of
magnetization shown by 70b.
The suitable selection of the bias magnetic field has the following
meaning. In said preliminary recording of first type, such bias
magnetic field is essentially unnecessary, since the magnetizations
of the first, second and third magnetic layers receive forces
(exchange forces) to orient the magnetizations thereof in
respective stable directions with respect to the magnetization of
the fourth magnetic layer 75. However said bias magnetic field is
provided, in a preliminary recording utilizing the second laser
power to be explained later, in a direction to assist the magnetic
inversion of the fourth magnetic layer 75. Also said bias magnetic
field should preferably have a same magnitude and a same direction
in both preliminary recordings with the first and second laser
power. In consideration of such conditions, the bias magnetic field
is preferably selected at a minimum necessary intensity required
for the preliminary recording with the second laser power to be
explained later, and such selection corresponds to the
above-mentioned suitable selection.
When the disk is heated close to the Curie point of the fourth
magnetic layer 75 by a second laser power (preliminary recording of
second type), the magnetization of said fourth magnetic layer 75 is
inverted by the above-mentioned bias magnetic field. Also the
magnetizations of the first, second and third magnetic layers are
oriented in stable directions with respect to the magnetization of
the fourth layer 75. Thus a record bit shown by 70d in FIG. 37 can
be formed from either state of magnetization shown in 70b.
Thus each area of the magnetooptical disk can have a preliminary
record bit 70c or 70d, shown in FIG. 37, by the bias field and by
the first or second laser power corresponding to the input
signal.
Then the magnetooptical disk 9 is further rotated, so that the
preliminary record bit 70c or 70d passes again through the magnetic
field generating unit 8, of which field intensity is selected, as
explained before, between the coercive forces of the magnetic
layers 73, 75. Thus the record bit 70c remains unchanged and
assumes a final record state 70e. On the other hand, the record bit
70d causes the inversion of magnetizations of the third and fourth
magnetic layers 74, 75 to assume another final record state
70f.
The aforementioned relations (18) to (22) are required in order
that the record bit 70f can stably exist.
The relations (20) and (22) are required in order that the second
and fourth magnetic layers assume an unstable state.
The relation (19) is required to cause a strong coupling between
the first and second magnetic layers, and to constantly orient the
magnetization of the first magnetic layer in a stable direction
with respect to the magnetization of the second magnetic layer.
The relation (21) is required in order to constantly stabilize the
magnetization of the third layer with respect to the magnetization
of either the fourth or the second magnetic layer.
Also the relation (18) is required in order to optimize the
recording sensitivity and the C/N ratio in reproduction, as will be
clarified in the following examples.
The record bit states 70e, 70f do not depend on the states prior to
recording but are solely determined by the recording laser power,
so that the over-writing operation is rendered possible. The record
bits 70e, 70f can be reproduced by irradiating the disk with a
reproducing laser beam and processing the resulting light with the
recording signal reproducer 7.
The intensity or modulation of the reproduced signal principally
depends on the magnetooptical effect of the first and second
magnetic layers as explained before. The present embodiment enables
recording with strong reproduction signal, since a material of a
high Curie point, namely of a large magnetooptical effect, can be
used for the first magnetic layer which receives the reproducing
beam.
EXAMPLE 34
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus, with ternary targets, and was rotated at
a distance of 10 cm from the targets.
A ZnS protective layer of 900 .ANG. in thickness was formed by
sputtering, in argon gas, from a first target with a sputtering
speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
Then a GdFECoTi alloy was sputtered, in argon gas, from a second
target with a sputtering speed of 100 .ANG./min. and a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer
of Gd.sub.18 Fe.sub.55 Co.sub.24 Ti.sub.3 with a thickness of 200
.ANG. and T.sub.1 =ca. 350.degree. C. H.sub.1 of said first
magnetic layer itself was lower than ca. 300 Oe, and the transition
metal was prevailing in the sub-lattice magnetization.
Then a TbFe alloy was sputtered from a third target under similar
conditions to obtain a second magnetic layer of Tb.sub.18 Fe.sub.82
with a thickness 150 .ANG. and T.sub.2 =ca. 140.degree. C. H.sub.2
of said second magnetic layer itself was 10 KOe, and the transition
metal was prevailing in the sub-lattice magnetization.
Then a GdTbFe alloy was sputtered from a fourth target under
similar conditions to obtain a third magnetic layer of Gd.sub.11
Tb.sub.11 Fe.sub.76 with a thickness of 100 .ANG. and T.sub.3 =ca.
160.degree. C. H.sub.3 of said third magnetic layer was 100-300 Oe,
and the rare earth element was prevailing in the sub-lattice
magnetization.
Then a TbFeCo alloy was sputtered from a fifth target under similar
conditions to form a fourth magnetic layer of Tb.sub.23 Fe.sub.72
Co.sub.5 with a thickness of 200 .ANG. and T.sub.4 =ca. 210.degree.
C. H.sub.4 of said fourth magnetic layer itself was 500-1500 Oe,
and the rare earth element was prevailing in the sub-lattice
magnetization.
Subsequently a ZnS protective layer of 2000 .ANG. in thickness was
formed by sputtering from the first 1/2 target under the same
conditions as before.
After said layer formations, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk.
Said disk was mounted on a record/reproducing apparatus, and was
made to pass through a unit generating a magnetic field of 2 KOe,
with a linear speed of ca. 7 m/sec. Recording was conducted with a
laser beam of a wavelength of 830 nm, focused to ca. 1 .mu.m and
modulated in two levels of 4 and 8 mW, with a duty ratio of 50% and
a frequency of 2 MHz. The bias field was 150 Oe. Binary signals
could then be reproduced by irradiation with a laser beam of 1
mW.
The above-explained experiment was repeated with a magnetooptical
disk already recorded over the entire surface. The previously
recorded signals were not detected in the reproduction, and the
possibility of overwriting was confirmed in this manner.
EXAMPLE 35
Samples of magnetooptical disk was prepared with the same materials
and structure as in the Example 34, with varying thicknesses in the
first to fourth magnetic layers. Results of measurements of
reflectance and Kerr rotation angle are shown in Tab. 17. The
reflectance was generally in a range of 23-24%, including the
reflection of ca. 4% from the surface of the substrate. Also a
larger Kerr rotation angle will provide a larger C/N ratio of the
reproduction signal.
TABLE 17 ______________________________________ h1 + h2 + h1 h2 h3
h4 h1 + h2 h3 + h4 .theta.k Sample (.ANG.) (.ANG.) (.ANG.) (.ANG.)
(.ANG.) (.ANG.) (.degree.) ______________________________________
35-1 50 100 100 700 150 950 0.70 35-2 100 100 100 700 200 1000 0.76
35-3 150 100 100 700 250 1050 0.83 35-4 200 100 100 700 300 1100
0.83 35-5 100 50 100 700 150 950 0.72 35-6 100 100 100 700 200 1000
0.76 35-7 100 150 100 700 250 1050 0.79 35-8 100 200 100 700 300
1100 0.79 35-9 100 300 100 700 400 1200 0.79 35-10 200 100 50 100
300 450 0.60 35-11 200 100 100 100 300 500 0.74 35-12 200 100 100
200 300 600 0.83 35-13 200 100 100 300 300 700 0.83 35-14 200 100
100 400 300 800 0.83 35-15 200 100 100 500 300 900 0.83 35-16 200
100 100 600 300 1000 0.83
______________________________________
The results on the samples 35-1 to 35-9 indicate that the Kerr
rotation angle .theta.k shows saturation close to a value for a
sufficiently thick first magnetic layer, if the total thickness
h.sub.1 +h.sub.2 of the first and second magnetic layers exceeds
250 .ANG., only when the total thickness of the first to fourth
magnetic layer is sufficiently large.
Also the results on the samples 35-10 to 35-16 indicate that the
Kerr rotation angle .theta.k shows saturation, if the total
thickness h.sub.1 +h.sub.2 of the first and second magnetic layers
exceeds 250 .ANG., when the total thickness of the first to fourth
magnetic layers is equal to or larger than 600 .ANG..
Therefore, in order to obtain a high C/N ratio at the reproduction,
it is preferable to select the layer thicknesses in such a manner
as to satisfy conditions h.sub.1 +h.sub.2 .gtoreq.250 .ANG. and
h.sub.1 +h.sub.2 +h.sub.3 +h.sub.4 .gtoreq.600 .ANG..
Other Recording Processes than Overwriting
In the foregoing embodiments, the magnetooptical recording medium
of the present invention has been explained in relation to an
overwriting process, but said medium can also be employed in the
conventional recording process consisting of recording, erasure and
re-recording. In such process there can be considered a case in
which the magnetizations of two magnetic layers are in a mutually
stable state in the recorded area, and another case in which said
magnetizations are in a mutually stable state in the unrecorded
area. These two cases will be separately explained in the
following.
(i) Case of stable state in unrecorded area
Recording is conducted on a magnetooptical recording medium of a
structure as shown in FIG. 3, with an apparatus as shown in FIG. 6.
The recording process is illustrated in FIG. 39, wherein it is
assumed that the magnetizations of the magnetic layers 2, 3 are
stable when they are oriented in a same direction (parallel).
At first the magnetizations of the disk 9 are oriented upwards as
shown by 80a in FIG. 39. An area to be recorded is locally heated,
by a laser beam of a recording power from the record/reproducing
head 5, to a temperature close to the Curie point of the second
magnetic layer 3. Simultaneously there is applied a bias magnetic
field (downward in FIG. 39) of a magnitude which is necessary, or,
preferably minimum necessary as will be explained later, for
inverting the magnetization of the second magnetic layer 3. Thus,
following the magnetic inversion of the second magnetic layer 3,
the magnetization of the first magnetic layer 2 is also oriented in
a stable direction (same direction in the present case) with
respect to the magnetization f the second magnetic layer 3. In this
manner a record bit 80b shown in FIG. 39 is formed.
Upon rotation of the magnetooptical disk 9, said bit 80b passes
through the position of the magnetic field generating unit 8, of
which field intensity is selected between the coercive forces
H.sub.L and H.sub.H of the two magnetic layers and of which field
is oriented in a direction to invert the magnetization of the
second magnetic layer in the bit 80b in FIG. 39, whereby the second
magnetic layer 3 is magnetized along the direction of the field
from said unit 8 as shown by 80c, while the first magnetic layer 2
retains the magnetization state 80b. In this manner the
magnetizations of the two magnetic layers are in a mutually
opposite or antiparallel state, and such state is utilized as the
final record.
The record bit 80c can be erased in the following manner. The
magnetooptical disk is rotated by a spindle motor, and the record
bit, in passing the record/reproducing head 5, is irradiated with a
laser beam of an erasing power, whereby the irradiated area is
heated close to the Curie point of the first magnetic layer 2.
Since the second magnetic layer 3 has a coercive force enough for
retaining the bit in stable manner at such temperature, the record
bit is erased as shown by 80d in FIG. 39 through a suitable
selection of the bias magnetic field.
The suitable selection of the bias field has the following
meaning.
In this erasing process, the bias field is essentially unnecessary
because the first magnetic layer 2 receives an exchange force to
orient its magnetization in a stable direction (same direction in
this case) with respect to the magnetization of the second magnetic
layer 3. However, as explained before, the bias field is provided,
for the recording process, in a direction to assist the magnetic
inversion of the second magnetic layer 3. It is convenient if said
bias field selected for the recording process can be maintained
also in the erasing process with same magnitude and direction.
Based on such consideration, the suitable selection of the bias
field is achieved by selecting an intensity and a direction
enabling the recording process and not hindering the erasing
process, preferably an intensity and a direction which are minimum
necessary for the recording process, and such bias field is then
maintained also in the erasing process.
However it is also possible to remove the bias field at the
erasing, since it is essentially unnecessary as explained before.
Also it is not necessary to maintain the bias field in the same
direction and magnitude in the recording and erasing processes, if
the record/reproducing head is capable of varying the bias field
according to the recording and reproducing operations. Since the
recording and erasing operations basically require mutually
opposite bias fields, and such variation of the bias field enables
recording are erasure at a higher speed.
Though FIG. 39 shows the principle of recording and erasure in case
the magnetizations of the first and second magnetic layers are
stable when they are oriented in a same direction, a substantially
same principle is applicable also if said magnetizations are stable
when they are oriented in mutually opposite or antiparallel
directions. Such principle is illustrated in FIG. 41, wherein the
states of magnetization 81a-81d respectively correspond to 80a-80d
shown in FIG. 39.
EXAMPLE 36
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with ternary targets, and was rotated at
a distance of 10 cm from the targets.
A ZnS protective layer of 1000 .ANG. in thickness was formed by
sputtering, in argon gas, from a first target with a sputtering
speed of 100 .ANG. and a sputtering pressure of 5.times.10.sup.-3
Torr. Then a TbFe alloy was sputtered, in argon gas, from a second
target with a sputtering speed of 100 .ANG. and a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a first magnetic layer
of Tb.sub.18 Fe.sub.82 with a thickness of 500 .ANG., T.sub.L =ca.
140.degree. C. and H.sub.H =ca. 10 KOe.
Then a TbFeCo alloy was sputtered, in argon gas, with a sputtering
pressure 5.times.10.sup.-3 Torr to obtain a second magnetic layer
of Tb.sub.23 Fe.sub.73 Co.sub.4 with a thickness of 500 .ANG.,
T.sub.L =ca. 180.degree. C. and H.sub.L =ca. 1 KOe.
Subsequently a ZnS protective layer of 3000 .ANG. in thickness was
formed by sputtering, in argon gas, from the first target with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After said layer formations, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk.
Said disk was mounted on a record/reproducing apparatus, and was
made to pass through a magnetic field generating unit of a field
intensity of 2.5 KOe, with a linear speed of ca. 15 m/sec., and the
recording and erasing were conducted with a laser beam of a
wavelength of 830 nm, focused to ca. 1 .mu.m. The recording was
conducted with a laser beam of 5 mW, modulated with a duty ratio of
50% and a frequency of 4 MHz.
The bias field was ca. 150 Oe, in a direction to invert the
magnetization of the second magnetic layer. Intensity and direction
of the bias field were maintained same, both at recording and at
erasing.
The erasure was conducted by irradiation with a continuous laser
beam of various powers.
The reproduction was then conducted by irradiation with a laser
beam of 1 mW. When the erasing laser power was equal to or larger
than 5 mW, the remaining unerased signal was less than 10 dB at the
reproduction and was saturated as shown in FIG. 40, and this level
corresponds to an almost complete erasure.
REFERENCE EXAMPLES 1, 2
A sample of Reference Example 1 was prepared in the same manner and
structure as in the Example 36, except that a ZnS intermediate
layer of 100 .ANG. was formed between the first and second magnetic
layers, which, in this case were magnetostatically coupled.
Also a sample of Reference Example 2 was prepared by forming the
first magnetic layer in a thickness of 1000 .ANG. only, instead of
forming the first and second magnetic layers, in a similar process
as in the Example 36.
These samples were tested in recording and erasing in the same
manner as in the Example 36. As shown in FIG. 40, the erasure
required a power of ca. 7 mW.
It was confirmed that, in the Example 36 and in the Reference
Examples 1 and 2, a signal with a C/N ratio of 45-50 dB could be
recorded with a laser power of 5 mW.
(ii) Case of stable state in recorded area
Recording is conducted on a magnetooptical recording medium of a
structure as shown in FIG. 3, with an apparatus as shown in FIG. 6.
The recording process is illustrated in FIG. 42, wherein it is
assumed that the magnetizations of the magnetic layers 2, 3 are
stable when they are oriented in a same (parallel) direction.
At first the magnetizations of the disk 9 are oriented as shown by
82a in FIG. 42. An area recorded is locally heated, by a laser beam
of a recording power from the record/reproducing head 5, to a
temperature close to the Curie point of the second magnetic layer
3. Simultaneously there is applied a bias magnetic field(upward in
FIG. 42) of a magnitude which is necessary, or, preferably minimum
necessary as will be explained later, for inverting the
magnetization of the second magnetic layer 3. Thus, following the
magnetic inversion of the second magnetic layer 3, the
magnetization of the first magnetic layer 2 is also oriented in a
stable direction (same direction in the present case) with respect
to the magnetization of the second magnetic layer 3. In this manner
there is formed a record bit as shown by 82b in FIG. 42. The bias
field in this state is however not necessary, since, in this
medium, the magnetizations are stable when they are oriented in a
same direction.
Said bit can be erased by passing the disk 9 again under the
record/reproducing head 5. Said area is locally heated, by a laser
beam of an erasing power, close to the Curie point of the second
magnetic layer 3. Simultaneously there is applied a bias magnetic
field (downward in FIG. 42) of a magnitude which is necessary,
preferably minimum necessary as will be explained later, for
inverting the magnetization of the second magnetic layer 3. Thus,
following the magnetic inversion of the second magnetic layer 3,
the magnetization of the first magnetic layer 2 is oriented in a
stable direction (same direction in this case) with respect to the
magnetization of the second magnetic layer 3.
Then the disk 9 is further rotated whereby the area 82c passes
through the position of the magnetic field generating unit 8, of
which field intensity is selected at a value between the coercive
forces H.sub.L, H.sub.H of the magnetic layers and of which field
is oriented in a direction to invert the magnetization of the
second magnetic layer 3 in the bit 83d, whereby the second magnetic
layer 3 is magnetized according to the direction of the magnetic
field from the unit 8, while the first magnetic layer 2 retains the
magnetization of the state 82c. In this manner the magnetic layers
show mutually opposite magnetizations, thus returning to the state
82a. The erasure is achieved in this manner.
Though FIG. 42 shows the principle of recording and erasure in case
the magnetizations of the first and second magnetic layers 2, 3 are
stable when they are oriented in a same direction, a substantially
same principle is applicable also if said magnetizations are stable
when they are oriented in mutually opposite or antiparallel
directions. Such principle is illustrated in FIG. 43, wherein the
states of magnetization 83a-83d respectively correspond to 82a-82d
in FIG. 42.
EXAMPLE 37
A pregrooved and preformatted polycarbonate disk substrate was set
in a sputtering apparatus with ternary targets, and was rotated at
a distance of 10 cm from the targets.
A ZnS protective layer of 1000 .ANG. in thickness was formed by
sputtering, in argon gas, from a first target with a sputtering
speed of 100 .ANG./min. and a sputtering pressrue of
5.times.10.sup.-3 Torr. Then a TbFe alloy was sputtered, in argon
gas, from a second target with a sputtering speed of 100 .ANG./min.
and a sputtering pressure of 5.times.10.sup.-3 Torr to obtain a
first magnetic layer of Tb.sub.18 Fe.sub.82 with a thickness of 300
.ANG., T.sub.L =ca. 140.degree. C. and H.sub.H =ca. 10 KOe.
Then a TbFeCo alloy was sputtered, in argon gas, with a sputtering
pressure of 5.times.10.sup.-3 Torr to obtain a second magnetic
layer of Tb.sub.23 Fe.sub.63 Co.sub.14 with a thickness of 500
.ANG., T.sub.H =ca. 200.degree. C. and H.sub.L =ca. 1 KOe.
Subsequently a ZnS protective layer of 3000 .ANG. in thickness was
formed by sputtering, in argon gas, from the first target with a
sputtering speed of 100 .ANG./min. and a sputtering pressure of
5.times.10.sup.-3 Torr.
After said layer formations, the above-mentioned substrate was
adhered to a polycarbonate plate with hot-melt adhesive material to
obtain a magnetooptical disk.
Said disk was mounted on a record/reproducing apparatus, and was
made to pass through, with a linear speed of ca. 15 m/sec., a
magnetic field generating unit of a field intensity of 2.5 KOe, and
the recording and erasing were conducted with a laser beam of a
wavelength of 830 nm focused to ca. 1 .mu.m.
The recording was conducted with the laser beam modulated with a
duty ratio of 50% and a frequency of 4 MHz, and with various
powers. The bias field was not used.
The erasure was conducted by irradiation of a continuous laser beam
of 6 mW, in the presence of a bias magnetic field of ca. 150 Oe in
a direction to invert the magnetization of the second magnetic
layer.
Reproduction was conducted with a laser beam of 1 mW. It was found
that the recording was possible with a power of ca. 3 mW, as shown
in FIG. 44.
EXAMPLE 38
For achieving a recording witha higher speed than in the Example
37, a bias magnetic field of 150 Oe was applied in a direction, in
which the magnetization of the first magnetic layer was oriented at
the recording. In the present example the direction of the magnetic
field was inverted at the erasing, because the direction of bias
field at recording is opposite to that at erasing. The necessary
recording power was ca. 2.6 mW.
EXAMPLE 39
Recording was conducted with a bias magnetic field, which was
maintained constant in direction and in intensity, both for the
recording and erasing. The magnitude of the bias field enabling
erasure and not hindering recording was ca. 200 Oe.
REFERENCE EXAMPLES 3, 4
A sample of the Reference Example 3 was prepared in the same manner
as in the Example 39, except that a ZnS intermediate layer of a
thickness of 100 .ANG. was formed between the first and second
magnetic layers. In this sample said layers were magnetostatically
coupled.
Also a sample of the Reference Example 4 was prepared in the same
manner, but by forming the first magnetic layer in a thickness of
800 .ANG., instead of forming the first and second layers.
Recording and erasing were conducted on these samples, in the same
manner as explained before. The power required for recording was
ca. 4 mW as shown in FIG. 44.
Erasure was proved with a laser power of 6 mW, on the samples of
the Example 37 and the Reference Examples 3 and 4.
EXAMPLE 40
Recording was conducted in the same manner as in the Example
37.
Erasing was conducted with the following method, which is suitable
for the erasure of the entire surface of the disk or the erasure of
plural tracks without excessive precision. More specifically, the
erasing was achieved by another magnetic field generating unit,
which was positioned between the recording head 5 and the magnetic
field generating unit 8 and which is capable of generating a
magnetic field of a direction opposite to that of the field
generated by said unit 8 and of a magnitude enough for inverting
the magnetization of the first magnetic layer. In this manner it is
no longer necessary to effect the erasure with the
record/reproducing head.
Improvement of the Apparatus
FIG. 45 is a schematic view of another embodiment of the
magnetooptical recording apparatus of the present invention,
wherein same components as those in FIG. 8 are represented by same
numbers and will not be explained further. In the present
embodiment, the first magnetic field generating means 24 is
replaced by a leading field of a driving magnet of an actuator 84
for moving the objective lens 19 of the optical head 14. Also the
second magnetic field generating means 25 is positioned at the same
side as the optical head 14, with respect to the disk 11. It is
therefore rendered possible to reduce the number of component parts
and to compactize the apparatus.
FIG. 46 is a partially cut-off plan view showing still another
embodiment, in which same components as those in FIG. 45 are
represented by same numbers and will not be explained further. In
this embodiment the second magnetic field generating means 85 is
mounted on the optical head 14, thereby achieving further
compactization of the apparatus.
The present invention is not limited to the foregoing embodiments,
but is subject to various modifications within the scope and spirit
of the appended claims.
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